The Stovebolt Site Forums The Shop Area Engine Shop Darn Idling!!!

So here is the scoop: I am trying to get my freshly rebuilt '51 235 to idle, but I can't seem to get the idle speed below 800 rpm and when I go below that, it runs rough and acts like it wants to cut out. The valve clearances have been triple checked and are correct, the engine is timed slightly advanced, and the idle mixture screw is 1.5 turns out (rochester 1 barrel). I did a vacuum test and at 1000 rpm my gauge is reading 13. So, all signs lead to a vacuum leak. Does this sound right and if so, what's the next step? I have an HEI dizzy with adjustable vacuum advance. Also, I have not done a compression test. Should I do this before I move on? I am noticing a popcorn like sound coming from the manifold side of the motor/carb and there is black soot in the tailpipe. Any advice would be greatly appreciated...

You have a vacuum leak or a carb. problem. If you have a plastic spacer under the carb you need gaskets on both sides of it. There is a pipe plug or wiper vacuum outlet on the side of the intake manifold. Make sure there is something in the hole. Remove the idle mixture screw & blast air into the hole. Turn the idle mixture screw about 2-3 turns out from bottoming & start the engine. Slowly turn it in till the engine starts to slow down. Then turn it about 1/2 turn out. If none of this works tear down the carb & put a kit in it. These carbs are basic & simple to work on.

It does sound like a vacuum leak although I was having similar trouble with my rebuilt 235. I was using a different flywheel and checked TDC was not the same as before. I needed to advance significantly. I set the engine at TDC (by looking in the #1 sparkplug hole)and marked the flywheel

You can spray some wd 40 on the intake manifold conections and carb base to see if the engine changes idle temporarily that could help you narrow down the leak.

You can also set the timing by vacuum. Advance the dizzy until max vacuum is achieved. You can do the same with the carb idle adjustment. A vacuum gauge is a very versatile tool.

Thanks so much for the tips! I have a plastic spacer, but only the top side has a gasket, so I think that's the culprit. I also try the WD-40 trick if the extra gasket doesn't work. Might be Monday till I can work on it again, but I'll keep you guys updated...

when i sent the carb out to get rebuilt on my 40 and put it back on i changed out the insulator for a nicer looking one. after i fired up the truck it was running as yours. i was baffeled. i knew the carb was good and the only thing i changed was the insulator. when i pulled the carb off to change out the insulator to the one that was on it, it is where i saw my blunder. added the gasket and all was good after that. it was leaking just enough not to allow it to come down to idle and run smoothly.

Well, guys, I added a gasket to the insulator so now there is one one each side. I am still only pulling on average 11 vacuum. I advanced the timing so far that I couldn't see the BB in the opening anymore with the timing light. When I bring it up to 2000 RPM, the vacuum reads 15 at the highest and when I bring it down to 800 RPM (the lowest it'll go before cutting out), it reads 11 again. What can I do next? This is killing me!

P.S. I still have a crackling sound in the carb. All the plugs are pretty nicely wet with gas (especially the 3rd and 4th cylinder). The washers that came with the plugs are pretty worn out... could that be it?

Dumb question but does your carb insulator and gasket have the grooves that expose the tiny holes in the bottom of the carb?

I had 2 different gaskets and the notches are in different places. Blocking those holes might not cause the issue you are having but the carb will act funny if both are covered.

Each different gasket I had covered a different hole. I matched the gasket to the insulator and got much better results.

Just a thought
Good luck
Rich

I think you got a carb issue ... Time to rebuild

The carb gasket has the grooves that line up with the hole on the bottom. Wish that were it. Looks like I'm going to be rebuilding a carb... yikes (does anyone know of a good step by step carb rebuild - I recall seeing a link for one here in the forums). Is there anything else I could possibly try before a carb rebuild? Also, at idle speed, should the vacuum still be 19 - 21?

Yes vacuum should 19 to 21 at idle

There is a great online tool for diagnosing problems with a vacuum gauge

I have the link on my other computer I will send it to you

A kind soul sent on this site sent it to me when I was having there issues


Rich

There is a chevy online manual site that has a lot of information including carb rebuilds links below

The carb kit should have pretty good instuctions and measuring tools included

http://www.carbkitsource.com/carbs/tech/Rochester/B-index.html
http://chevy.oldcarmanualproject.com/

here is the vacuum tool as well

Good luck
http://www.secondchancegarage.com/public/186.cfm

I wonder if you dont have some plug wires crossed.

I have the plugs set 1 5 3 6 2 4... made that mistake at the beginning. Thanks for all the tips guys. I'll keep at it and post what I discover.

A quick carb question before I take it apart and rebuild: With the engine running as close to idle as I can get it, what should I see gas wise down the carb neck (i.e. a constant drip from the venturi, a slow drip, a spray?). I am beginning to think my float level is off and the engine is getting too much gas (the 3rd and 4th cylinder seem to be flooding... gas on piston). The carb wasn't rebuilt too long agon, but did sit for awhile. Thanks again...

Rebuilding the carb is a breeze. Just go slow and the instructions that come with the rebuild kit are suficiant. Just make sure the accelerator pump is the same when u purchase the kit. A auto or manual choke carb changes the pump and they won't let u return or swap the kit once opened. It's clear plastic package, so just check b-4 u open. I just did my first one and found that out.

That carb is pretty simple. Dont try to take the butterflys out of the shafts. It is not necessary. Check the float. Often they leak gas into the pontoon & get heavy. Shake it to hear any sloshing.

I appologize for going off on a tangent, but two items have been discussed in this thread that are also causing me problems ...
('54 3100 235 Rochester B)

1) Carb Insulator: My carb did not have an insulator. Bought one from Chevs of the 40s. The dimple on the insulator does not match the carb and gasket. Insulators on other vendors' sites appear to be the same. Should I route out a new dimple, or install it as-is with the correct gasket between it and the carb?

EDIT ... The mold marks on the insulator will be under the cutouts in the gasket. Only leaves a sliver of solid material on the outer edge of the insulator - possibly outside the edge of the carb. Looks like a vac leak just waiting to happen. Am I correct?

EDIT #2 ... "Village_Idiot" just posted a pic of the carb base on his '59 235 BC. The hole is positioned to work with my insulator from Chevs of the 40s. The hole on my '54 carb is closer to the mounting hole. Anyone know where to get a correct insulator for my '54 Rochester B?

2) Accelerator Pump: Panhead 59 noted that accel pumps for Rochester "B" and "BC" are different. This is the type that was in my "B" carb when I bought the truck and is the only pump in the NAPA Jiffy kit. The pump rod travels in the slot and only pushes the pump down about 3/16". Sometimes pumps OK; sometimes dribbles. Is there a better part?

Thanks,

I just ordered the kit. Should be here Tuesday. I'll let you guys know how it goes. LonnieC, I have the same problem with my insulator gasket. It has four cutouts (although the Rochester B only needs one) and they are right on the edge of the outer base. Vac leak waiting to happen is right...

Two questions: With the engine running as close to idle as I can get it, what should I see gas wise down the carb neck (i.e. a constant drip from the venturi, a slow drip, a spray?).

And

If I am only pulling around 11 vacuum with the engine at 800 rpm (as low as it'll go right now), should I assume the carb rebuild will take care of the vacuum issues, or should I still be looking for a vacuum leak? If so, where are the most common places to check?

Thanks again, guys.

Along with my previous post, I was wondering what the best procedure is for performing a valve job. I've been using this procedure:

http://www.speedprint.com/deves50/valveadjust.php

For top dead center, I've been going off the line between the UC on the flywheel. Also, I absent mindedly put my old spring shims in with the new springs I installed on the head. Could this effect my valve timing?

with all the other things you got going on and checking have you ruled out that it isnt a vacuum leak somewhere?

With the engine at idle, you shouldn't see any fuel going down the carb, or at the most, a very light spray mist. If you have drips or heavy spry, you are over fueling. With 3 and 4 cylinders wet, it sure sounds like a carb problem since those two holes are the first to receive fuel.

Checking the valves can be done a number of ways, I like to turn the engine till both valves are off the cam (closed) on the first cylinder. I set the to the proper spec then turn the engine so the next cylinder has closed valves, an so on till all six are done. I find it easier for me and I know each one was done the way it should be. I do end up turning the engine more times, put I'm OK with that.

Don't get hung up on the timing marks, just set it where it runs the best till you can get the whole package working together.

While the carb is off soaking in cleaner, recheck the valves. Also check the hold down bolts on the intake to head and intake to exhaust manifold. Plug off all vacuum lines except to the vacuum advance. Check the vacuum advance for a leak, suck on the hose and see if it holds vacuum.

You need to eliminate all possible reason for a leak or problem before moving forward.

Joe

I am not positive I've ruled out a vacuum leak. Where are the top areas to check for vac leaks besides the intake manifold and carb gaskets? Also, I am hooking my vac gauge up to the port on the side of the manifold that connects to my hydrovac brake booster. Is this correct? Should I have the vac advance from the distributor disconnected and plugged too? Thanks again, guys! Hang in there with me...

i had an issue one time on the ashton with a vacuum leak. i disconnected everything that ran off of vacuum and pluged it. i knew it wasnt the vac. advance because mine doesnt have one. so that only left the carb mounting and the intake manifold. it ran fine. then i started hooking things back up one at a time until i found out what it was. wipers, hydrovac, two speed rear, fire pump, pcv valve. its been years since i did this that i cant remember what it was.

Thanks for the tip, 51 ashton. I will disconnect it all, then hook everything back up one at a time until I find the vacuum leak (if there is one).

Regarding the valves again: When I set my engine to TDC (the line between the UC mark), both intake and exhaust valves on cylinder one are closed, and both intake and exhaust valves on the number six cylinder are open. My question is, should the exhaust/intake on cylinder six both be open at the same time, or should there always be one open and one closed (same goes for the rest)...

There is always a slight overlap when the exhaust valve is not quite closed and the intake is just started opening at TDC.