1970 Rochester Carburator

[Terry B.]

The primary throttle shaft on my 1970 350/350 Rochester Carburator appears to sticking slightly near idle speed. When I slowly release the accelerator, the idle speed will only come back to around 1200rpm. If I rev up the engine and quickly release the accelerator pedal, the idle speed will return to around 750 rpm. When I disconnect the accelerator cable from the carburator, I get the same results i.e. if I manually slowly release the throttle shaft after accelerating, it will not return fully to idle. If I release it quickly, it will return to idle speed. I have tried a new accelerator return spring that connects between the primary throttle shaft and the accelerator cable bracket but that didn't seem to fix the problem. If I gently pull on the spring attaching the primary throttle shaft to the accelerator cable bracket, the idle speed will come back to it's normal 750rpm.

Any suggestions or ideas?

Terry

[Terry M.]

Re: 1970 Rochester Carburator

Terry,
I know this will get confusing with so many Terry's in the thread.

Does your carburetor have the dashpot that bears against the throttle linkage? The part near the bottom of the carb near the front mounting bolt in this picture?
That is a 1970 Rochester only feature designed to slow the throttle plates closing suddenly on deceleration and starving the engine of air -- causing it to momentarily run rich. In short it is designed to limit the sudden closing of the throttle plates.

1970 #28(2).jpg

[Terry B.]

Re: 1970 Rochester Carburator

Terry, Thanks for the reply

Yes, it does have the dashpot however, I don't think this is the problem because after slow release of the accelerator, the idle speed will remain at around 1200rpm forever unless I hit the accelerator quickly then release (sort of "goose" it if you will). After doing so , the idle speed will come back to 750rpm...hope that makes sense.

[Terry M.]

Re: 1970 Rochester Carburator

Terry, Thanks for the reply

Yes, it does have the dashpot however, I don't think this is the problem because after slow release of the accelerator, the idle speed will remain at around 1200rpm forever unless I hit the accelerator quickly then release (sort of "goose" it if you will). After doing so , the idle speed will come back to 750rpm...hope that makes sense.

Could the dashpot be malfunctioning and/or maladjusted? I believe it can be backed out or even removed just to be sure.
Otherwise it could be that your throttle shaft is binding, although the spring changes you made should alleviate that. Any chance there is some other way air is getting into the intake? Take off the vacuum hose to the vacuum system at the intake and plug that pipe to eliminate a leaky vacuum system.

[Terry B.]

Re: 1970 Rochester Carburator

Good suggestions, I'll try them both. Just to clarify, you're talking about plugging the vacuum pipe at the front of the Carb right?

[Clem Z.]

Re: 1970 Rochester Carburator

could be cause by a worn baseplate bore where the throttle shaft goes thru.

[Terry M.]

Re: 1970 Rochester Carburator

Good suggestions, I'll try them both. Just to clarify, you're talking about plugging the vacuum pipe at the front of the Carb right?

No Terry, I was talking about the vacuum fitting on the intake that goes to the HVAC and headlight system. It can be seen at the far right of the posted photo. The hose with the round white filter on it. In the photo the second hose that goes to that silver colored fitting is for the automatic transmission, which could be another source of a vacuum leak, if your car has an automatic transmission. I was aiming at eliminating a vacuum leak from a source other than the carburetor itself.

[Jim T.]

Re: 1970 Rochester Carburator

Did all 1970's come with the dashpot? My original owner 350/300 turbo 400 did not have the dashpot in your picture when I bought it. My 70 was built in July.

[Edward J.]

Re: 1970 Rochester Carburator

Terry B., I have seen this problem many times over the years, sometimes the rebuilders re-bush the throttle body and it may be a little to tight causing the throttle shaft to only bind when engine is running. this is because the vacuum under the throttle blades won't let the blade rest against the idle stop. And the same can happen as Clem says a badly worn throttle shaft or throttle body does the same. What you can do is add a slightly stronger return spring this generally helps.

[Clem Z.]

Re: 1970 Rochester Carburator

Terry B., I have seen this problem many times over the years, sometimes the rebuilders re-bush the throttle body and it may be a little to tight causing the throttle shaft to only bind when engine is running. this is because the vacuum under the throttle blades won't let the blade rest against the idle stop. And the same can happen as Clem says a badly worn throttle shaft or throttle body does the same. What you can do is add a slightly stronger return spring this generally helps.

you are better off to rebush the throttle base plate with teflon instead of brass. most of the newer FI throttle bodies are bushed with teflon. also holley uses teflon as bushing in some of its carbs. that green color you see on some throttle shafts is a teflon coating but it is thin and wear thru

[Stuart F.]

Re: 1970 Rochester Carburator

Generally speaking, QJ's have a cast iron base and therefore don't wear as much as say an AFB Carter with aluminum. I would expect them to go well over 100k miles before noticing any appreciable wear. I have one AFB that I know has about 70K on it and it is still tight, so it takes a lot of use before shaft bore wear presents a problem. I'd look elsewhere even to considering that someone tried to remove the throttle plates and didn't reseat them right. However, when wear does occur, it is usually at the bore behind the throttle arm linkage which is where most of the pull/push of the linkage and return spring happens. This then can cause the LF throttle plate to bind. Every AFB I have had needed an additional booster spring to achieve consistent throttle return to idle. AFB's, being aluminum, tend to react to the engine heat cycles more than QJ's or Holley's with cast iron bases.

FWIW

Stu Fox

[Clem Z.]

Re: 1970 Rochester Carburator

Generally speaking, QJ's have a cast iron base and therefore don't wear as much as say an AFB Carter with aluminum. I would expect them to go well over 100k miles before noticing any appreciable wear. I have one AFB that I know has about 70K on it and it is still tight, so it takes a lot of use before shaft bore wear presents a problem. I'd look elsewhere even to considering that someone tried to remove the throttle plates and didn't reseat them right. However, when wear does occur, it is usually at the bore behind the throttle arm linkage which is where most of the pull/push of the linkage and return spring happens. This then can cause the LF throttle plate to bind. Every AFB I have had needed an additional booster spring to achieve consistent throttle return to idle. AFB's, being aluminum, tend to react to the engine heat cycles more than QJ's or Holley's with cast iron bases.

FWIW

Stu Fox

QJs have always had aluminum throttle base plates and holleys since the late 50s. the cast iron ones were used on carter WCFBs and rochester 4 jets

[Joe L.]

Re: 1970 Rochester Carburator

QJs have always had aluminum throttle base plates and holleys since the late 50s. the cast iron ones were used on carter WCFBs and rochester 4 jets

clem-----


Yup. The Rochester carbs with the cast iron throttle bodies were the 4GC models (which, incidentally, were never used on a Corvette).

[Joe L.]

Re: 1970 Rochester Carburator

Did all 1970's come with the dashpot? My original owner 350/300 turbo 400 did not have the dashpot in your picture when I bought it. My 70 was built in July.

Jim------


I don't know. However, I can tell you that I've searched several times for the part number of this "dashpot" and I've never been able to come up with it.

[Terry B.]

Re: 1970 Rochester Carburator

That's my fear also, may have to take it to a carb specialist and have him check it out...appreciate the reply