Rochester Guru?

[jonnyc]

Due to problems, I rebuilt my Rochester carb today. All seemed to go well, but I can't get it to run right. It has a rough, surging idle. The idle screw is almost all the way in and the idle isn't too high at all, and the mixture screw doesn't seem to have much effect or change much, all they way in or even 5 or 6 turns out. Just to be clear, I have rebuilt Rochesters before with no ill effects.
Any idea of what I may have screwed up or what the problem is? Probably have to tear it down again, but I'd like any ideas before I do.

[swray]

Throttle shaft sucking air?

[TeriAnn]

Two thoughts immediately come to mind. One is the wrong gasket under the carb and the other is the wrong pressure spring on a check ball. Most of the new kits were designed for either single or dual barrel and have a stronger spring that originally came on the early single barrel carbs.


The early Rochester B series carburetor has a hole in the base. This is the direct manifold vacuum passage which operates the sustained power system. This system provides additional fuel for sustained high speed operation or increased road load power It also helps with fuel economy. The bottom line here is that you do not want to cover this hole with a gasket.

Rochester base gaskets have a cut out slot going between this hole and the throat of the carb at the base. The Land Rover carb base gasket does not. Use the Rochester base gasket, NOT the LR carb base gasket.

Most auto parts stores carry a thick fiber spacer that fits between the Rochester carburetor and the intake manifold. This spacer helps keep the carburetor from getting too hot and vapour locking. It also provides an air passage between the hole in the base and the throat of the carburetor. Use this fiber spacer instead of the Land Rover spacer.

If you can not find the fiber spacer you can use three Rochester carb base gaskets stacked together on top of the Land Rover spacer to provide insulation and space for the vacuum flow.


There is a pressure valve inside the carburetor body that consists of a steel ball and a spring. There is a metal rod across the top of the opening to keep the parts in place. A rebuild kit comes with a new ball and spring. Be advised that the spring rate is different than the original and replacing it can cause problems. Leave this pressure valve alone if you can when rebuilding the carburetor If you need to open up the passage reuse the old spring.


Float level (base of carb top to bottom of float, carb top upside down) = 1-9/16" using Rochester float gauge M-250. Bend float arms to adjust.

Float drop (base of carb to to bottom of float, carb top right side up) = 1-3/4" using Rochester gauge BT-93. Bend float tab at float valve to adjust.

Hope this helps.

[jonnyc]

The fuel inlet on my original airhorn was stripped, so I used another for the rebuild. All the bits were new, but I didn't remeasure the float levels. I will do that.

I did use the slotted Roch. gasket, but perhaps it isn't thick enough. I will stack it with the old one for some extra thickness.

I did pull that spring and check ball out for cleaning but I reused the old spring.

The only change I made was to replace the 53 jet with a 51 that I had. I thought that would be a proper change.

I'll block the throttle shaft and see if that has any effect.

Thanks!

[mearstrae]

Also, check the idle mixture screw, sometimes the pointed tip breaks off and causes idle troubles.

'95 R.R. Classic LWB
'76 Series III Hybrid 109
'70 Rover 3500S

[jonnyc]

Nah, mix screw is fine, and I even swapped it for a spare to see if there was any change.
Took the carb off again, will retrace all my steps tomorrow.

Oh the joys of old Rover ownership.

[Lance]

Are the problems you are having now different then the problems you had before the carb rebuild?

[jonnyc]

Don't know yet. Even as we "speak", the Rover is warming up so I can see if the carb will adjust properly.

[jonnyc]

Results not what I had hoped.

1. Rough idle with some "popping".
2. Idle too high on driving test, but if set lower it stalls.
3. Mixture screw in close to all the way, can't seem to find a high, smooth setting.
4. I had good vacuum before rebuild (18-20), now it is bouncing between 6 and 8. No changes were made to the bottom of the carb, I did put on a different air-horn. All seems solid and correct.

Thoughts?

[Lance]

If you put your hand on the intake of the carb. and slowly start to block off the air does anything change? What do the plugs look like, black or normal?

[bkreutz]

Results not what I had hoped.

1. Rough idle with some "popping".
2. Idle too high on driving test, but if set lower it stalls.
3. Mixture screw in close to all the way, can't seem to find a high, smooth setting.
4. I had good vacuum before rebuild (18-20), now it is bouncing between 6 and 8. No changes were made to the bottom of the carb, I did put on a different air-horn. All seems solid and correct.

Thoughts?
Somethings wrong here, either you have a vacuum leak or sticky valves (or extremely retarded timing). If you haven't messed with the timing then the first two is where I would be looking (vacuum leak first, my old rule is check the easy stuff first, you might throw a timing light on to verify the timing just to eliminate that for sure). Here's a good reference site regarding interpreting the vacuum reading. http://www.secondchancegarage.com/public/186.cfm

[jonnyc]

OK, kind of odd, you guys may be on to something.

1. As I slowly closed off the air intake the engine rev'd way up, not what I expected.

2. Here are the plugs:
http://i141.photobucket.com/albums/r46/jonnycpics/plugs002.jpg
http://i141.photobucket.com/albums/r46/jonnycpics/plugs001.jpg
http://i141.photobucket.com/albums/r46/jonnycpics/plugs003.jpg

To add: I haven't touched the timing. My rule of thumb is that if it starts right up, accelerates fine, and stops right away...the timing is pretty close.

[Lance]

Don't suppose you have any other carbs around you could try just to see what happens?
Do you think you are getting enough fuel to the carb? Dirty filter or screen in the fuel pump bowl?

[Sputnicker]

The only change I made was to replace the 53 jet with a 51 that I had.

Have you tried the 53 jet? If it's worse than before you rebuilt it, then I'd look first at anything that's different. I'd probably take it back apart and make sure something didn't get put together wrong (we've all been there). It could also be a blockage somewhere, so try blowing out jets and passages with compressed air. The plugs look too lean.

[jonnyc]

This last time I thought of that and put a 52 in. Guess I could go back to the 53.

[baja gs]

I didnt see if you have you checked for vacuum leaks. If not, spray brake cleaner around the carb gaskets, if rpms surge, you have a leak.:confused:

[jonnyc]

It is a bit leaky around the carb shaft, but nothing that I can find that wasn't there before.

[Lance]

I had a problem with my 109 that took me months to figure out, but the short version is there was a hole in the intake manifold where the two manifolds connect,up inside the intake manifold but on the exhaust side. The exhaust had eaten away the aluminum, you can't reach it to test it for leaks. I removed the intake manifold, plugged the intake hole were the carb. goes then slowly poured water in the manifold & sure enough water started leaking out of that black hole were the exhaust goes. Can't see it & there is no way to leak test it because of it's location. I don't know how common this is on Rovers, I haven't been around them very long but it took me about 6 months to find this. I fixed it for now by making a plate to go between to two manifolds.

[willincalgary]

I had a problem with my 109 that took me months to figure out, but the short version is there was a hole in the intake manifold where the two manifolds connect,up inside the intake manifold but on the exhaust side. The exhaust had eaten away the aluminum, you can't reach it to test it for leaks. I removed the intake manifold, plugged the intake hole were the carb. goes then slowly poured water in the manifold & sure enough water started leaking out of that black hole were the exhaust goes. Can't see it & there is no way to leak test it because of it's location. I don't know how common this is on Rovers, I haven't been around them very long but it took me about 6 months to find this. I fixed it for now by making a plate to go between to two manifolds.

My truck has always had some indications of a vacuum leak. It lopes on idle when choked and has a persistent miss despite having stellar compression, new cables, good timing, new properly gapped plugs and an electronic distributor. I have tried everything imaginable to find a vacuum leak as whenever people get under the hood they immediately suspect one. Never had any luck. I will have to check on this.

[Lance]

Willincalgary: My simptoms started out of the blue & I had a misfire on two cylinders. I to tried everything I could think of to make it run. Check your manifold, It's pretty easy to remove.

[jonnyc]

Well, this is getting frustrating.
I checked everything I did to the carb, put a bigger jet back in, ran 2 doses of upper cyl./head cleaner/lube, hunted for any possible vacuum leak.....and it's still running worse than before I rebuilt the carb.
Timing and ignition system are good, fuel system from tank to carb inlet are good and clean.
Idle is still too high, surges a bit, and is too rough and "poppy".
I really appreciate the input so far, but I feel I need more. It's too cold to start with the manifolds, some I'm looking for some quicker ideas.

[Lance]

Did you ever find anything? Fixed?

[jonnyc]

No, not yet. Weather's supposed to be nice tomorrow, so I will go through all I can to solve the problem. I'm leaning towards some kind of major vac leak, or a few small ones. If not the hoses, it surely must be someplace between the head/manifold joints and the carb air-horn. A semi-intelligent guy like me should be able to find the problem(s).

[Lance]

Thats what I thought with my problem...I no dummy. My bet is on the intake manifold leaking at the bottom where the exhaust goes through. It's in a location that can't be tested and we would never find.

[jonnyc]

Which is why today I ordered a set of manifold gaskets......just in case.:thumb-up:

[Lance]

Good luck.... hope you find it. I'm new to these Rovers but I have learned alot in a few months.

[jonnyc]

OK, checked every source for a vac leak and I think I have lots of little ones adding up to a big one. Engine rev'd when sprayed at:
-head-manifold joints (mainly at the exhaust ports?)
-engine side of the int/exh manifold joint
-front of block betw manifold and carb
-front of carb base
-ends of throttle rod

Manifold gasket set is on the way, so I will take the manifolds off and do everything I can do to tighten up all the seam. I will also look for any problems in the int/exh manifold joint.

I hope all this solves my issues.:(

[jonnyc]

Lance,
You're saying your leak was INSIDE the intake manifold and invisible from the outside, not at the joint between the two manifolds?

[Lance]

Thats correct. Not sure how common this is. I took my manifolds off the engine, seperated the two, poured water slowly in the void "under" the intake where exhaust heats the intake & water came out the inside of the manifold. Then I plugged the hole where the carb bolts on and slowly poured water in the intake ports and water came out the exhaust void. (not sure what the correct terminology is for that void/cave on the intake). My Rover always ran great, one day I got in it to go for a drive and it ran terrible. It didn't happen while I was driving but sitting over night???

[jonnyc]

***UPDATE***
Weather and schedule cooperated today and I got a chance to start it up and see if I fixed anything.

1. Large number of small vac leaks seem to be gone, probably due to new gaskets and a bit of judicious sealing and tightening.
2. Vac gauge is showing too low, bouncing in time with the idle between 6 and 10.
3. Idle is a little better, still sort of rolling and surging, backfiring seems almost gone.
4. Mixture screw can go all the way in without stalling or going very rough.
5. Idle screw has to be almost all the way in to prevent stalling.

I found no cracks, leaks or apparent damage in either of the manifolds when they were off.
Next???

[bkreutz]

Wow, what a puzzle. Your vacuum readings are obviously too low, considering what you've already checked the only other things I can think of is a restricted exhaust (not likely by the looks of your plugs, but it can't hurt to check, the idea is that if you can't exhale, you can't inhale) Another thing to check is valve timing, it's possible that the valve timing has jumped which could result in low vacuum. Other than these two things, I'd start re-checking things again. There's something amiss, just got to find it. ;)

[jonnyc]

The exhaust system was sitting unused for about 5 years along with the rest of the truck, might be something there. Any way to check that? Block the end and see if there's any change?
When you say a change in valve timing, are you saying that the chain might have slipped? That's not a job I want to do again any time soon.

[bkreutz]

The exhaust system was sitting unused for about 5 years along with the rest of the truck, might be something there. Any way to check that? Block the end and see if there's any change? You should feel sufficient pressure on your hand covering the tailpipe. (not very scientific but will show a problem)
When you say a change in valve timing, are you saying that the chain might have slipped? (more accurately jumped a tooth or two) That's not a job I want to do again any time soon.
Is this something you've already done?, how recently? Considering the time you've spent with this it might be worthwhile to make sure that it's still in proper time.

[jonnyc]

The exhaust check....never.
Timing chain....2 years or so before the truck got garaged over an ignition problem (and laziness).

However, it starts quick and shuts off quick, so the timing doesn't seem to be off. That's always been my rough-indicator-rule-of-thumb for timing.

[bkreutz]

Valve timing and ignition timing are entirely different things. I've seen engines with jumped valve timing start normally. May not be your problem but if it was me I wouldn't assume anything at this point.

[milhouse]

Does your master cylinder have a brake booster? Perhaps the booster is bad and the vacuum leak is there? You should be able to easily check that by removing the line to see if there's any change in the engine idle, or seeing if there is any difference in pedal firmness when the engine is running or not.

[jonnyc]

I disconnect the booster when I attach my vac gauge, so that isn't an issue.

bkreutz, I guess I have learned something new. Is there a way you know of to check or adjust valve timing without ripping apart the whole front end? I hear "timing" and I automatically think of ignition timing.

[bmohan55]

An old trick and old mechanic showed me is to hold a dollar bill in front of the exhaust pipe. It should constantly be blowing away from the pipe, if it occasionally gets sucked back towards the pipe it indicates that a valve is sticking open or a vacume leak. YMMV!!!

[bkreutz]

I disconnect the booster when I attach my vac gauge, so that isn't an issue.

bkreutz, I guess I have learned something new. Is there a way you know of to check or adjust valve timing without ripping apart the whole front end? I hear "timing" and I automatically think of ignition timing.
Not that I know of on a LR engine. (I do know how to do it on a Porsche flat 6 though, but not much help in this situation :rolleyes:)

[printjunky]

I'm still with the vacuum leak crowd, a bit. But if you've sprayed starter fluid all around any possible ingress points and gotten no rev, whatsoever, then you're right, you plugged 'em all. But I would re-triple check this simple thing before tearing the front cover off. It's just that about 50% of the time, I think I've entirely eliminated the simple stuff, moved on to the major things, and in the end find out I in fact did miss something when I was checking the simple stuff. (I can sometimes be too quickly resort to "The mechanical sky-is-falling!") This is why I now have significantly robust and redundant battery/engine/chassis grounds - too many times thinking I had good ground, quintuple-checking and meter testing the grounds, only to find that it was still a simple ground problem, and there was no need to have removed and disassemble/tested my starter (alternator/relay, etc).

The only other times I've has any similar issues that wasn't a vacuum leak, was when I had a tiny (TINY) piece of something stuck on my needle valve in the Rochester. It only came off when rubbed with a q-tip (in other words, it didn't just flush away). Fuel was just dumping into the intake.

And once I had a funky acting intermittent mechanical (original) fuel pump that caused some fine-starting, bad running conditions.

And last summer, right before I had a catastrophic failure as the top timing gear locker disintegrated and ruined the cam. Before the failure, the cam was not being held in position, so my valve and ignition timing was being affected. Not to say you have this exact same problem, but to corroborate other's proposed valve timing diagnosis.

But none of these three examples was quite as exact a match to your symptoms as the vaccuum leaks I've had.

[bkreutz]

I agree, I'm still thinking vacuum too. But after all this time and messing around, I wouldn't rule anything out. Taking the front of the motor apart is a real pain, but there has to be an answer. Maybe have a second set of eyes go through the obvious stuff again.

[Lance]

One thing you might try before getting in the motor is to make a metal plate and install it between the exhaust manifold and the intake. Basiclly isolating the two manifolds. It worked for me.