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:




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.