News:

Due to a technical issue, some recently uploaded pictures have been lost. We are investigating why this happened but the issue has been resolved so that future uploads should be safe.  You can also Modify your post (MORE...) and re-upload the pictures in your post.

Main Menu

Slight surging at low throttle pressure on 1953

Started by Ronmcfarland, November 14, 2020, 08:25:38 PM

Previous topic - Next topic

0 Members and 1 Guest are viewing this topic.

Ronmcfarland

Hi, have been having trouble with slight surging with low throttle pressure. Has anyone had this problem. Carburetor has been rebuilt. Thanks for your help. Ron

James Landi

Hi Ron,

May I suggest a "deep dive" into the wonders of a vacuum gauge.  I had a similar problem with an engine that's much newer than yours,however,  basic  8 cylinder technology and challenges of this sort can often be diagnosed with a vacuum gauge attached to the  intake manifold.  Research what these instruments will diagnose... they're quite remarkable, and will take a lot of the guess work out of the challenges, and often lead to a quick solution.  Hope this helps,  James

wrench

#2
Carb has been rebuilt may or may not be at issue.

A question would be ‘What is the state of tune of the car?’

Did the issue arise suddenly after the car was running fine? Or was the carb rebuilt because the car was ‘surging’ as you describe?

Could be on the ignition side and that is why I ask about the tune.

Could be mechanical which is why James suggested the vacuum gauge.

I would suggest to start at the beginning and describe when it started and what has been done so far. Also, a statement about the originality of the motor, mileage and any mods to it would also be helpful.

There are other tools and techniques depending on how far down the rabbit hole you want to go.

When you say ‘slight’ what does that mean? Is this an issue you want to pursue?

What is your level of technical ability?
1951 Series 62 Sedan
1969 Eldorado
1970 Eldorado (Triple Black w/power roof)
1958 Apache 3/4 ton 4x4
2005 F250
2014 FLHP
2014 SRX

James Landi

Jim has all good suggestions for additional input, should you decide to share more detail. SO many very minor worn parts or miss adjustments can cause irritating functions, especially at idle.  So, we're standing by for more input, and should you be "handy" and wishing advice from afar, we;re here.  James

Ronmcfarland

Hi Guys, thanks for getting back to me. I’ll give some background info. I had carb professionally rebuilt 2 & 1/2 years ago. Car was at our vacation home so didn’t’ get driven very often. I recently brought it home. I have replaced plugs wires, cap, rotor points condenser all good. Changed fuel filter and replaced electric fuel pump. I took car back to rebuilder, he took it apart and found some silt. He cleaned it replaced acc. Pump. Reinstalled and had same problem. Surging at low throttle pressure and would go away if I increased pressure slightly ( not enough to hit secondaries. Did it at all speeds. I brought it back and he reamed out supply tubes to idle jets and raised float level 1/16th “ to factory setting. Reinstalled and now only surges after 50 mph. I’m not sure if float level or reaming out fuel ports a .001 made it better. I have sprayed berrymans around manifold carb etc. no leaks. The only thing I need to do again is disconnect vacuum to advance as I used my hand vacuum pump to check advance and could not build up any pressure. I did disconnect it at carb but not sure I plugged carb vacuum completely . Need to try again. I first tried to put a piece of brake line but flair on end is too small to seat it carb. I have art of rubber capss and plugs but not sure plug I used sealed it completely. I am going to come up with another way. Maybe I can flair a piece of line the sme as flair on existing line. The brake line is double flares and too small. Sorry for the length of this but wanted to bring you guys up to speed. Thanks again

Daryl Chesterman

Ron, sorry but I question the "professional" rebuilding of your carburetor.  How would silt get into the carburetor if there was a fuel filter in place?  The fact that you were not able to get the vacuum advance to hold a vacuum indicates that there is a vacuum leak in its diaphragm.  With this leak, it is not possible to properly set the carburetor idle air adjustment or the idle RPM.  It is also possible that the static timing is not correct since the way it is set is by disconnecting the vacuum hose at the vacuum advance and plugging the hose (I use a small tapered punch or you can use a golf tee or other suitable object to completely plug the line) before setting the static timing.  The timing specification is at a certain idle speed, so it is best to set the idle air adjustment and idle speed at this time.  All of these settings should be on a thoroughly warmed up engine.  The vacuum advance should be replaced so that there will not be a vacuum leak when the static timing is set and the vacuum line reconnected to the advance.  While you have the distributor cap off, it is a good time to check that the centrifugal advance is working correctly.  You should be able to grasp the rotor and rotate it a few degrees either clockwise or counterclockwise against spring tension and when you release it, it should return to its original state.  If it doesn't rotate, or spring back from the rotation, this indicates that either the springs are broken or very weak, or the mechanism is sticky and needs to be cleaned and lightly lubricated.  The centrifugal advance will definitely affect engine performance, as will a defective vacuum advance.  Once you get any issues fixed regarding the timing, and have the idle air and idle speed adjusted properly, and you are still experiencing surging, then you will have to look for other problems with the carburetor rebuild.

You can use a light to shine down into the throat of the carburetor and see if you can see gas dripping down into the venturis with the engine idling.  If so, that is your problem causing the surging.

Daryl Chesterman


Ronmcfarland

HiDaryl since I posted the last time I was able to seal off vacuum advance from port in carburetor. I put my hand vacuum directly into the advance and it is definitely bad. My experience with vacuum advance not working is sluggish until mechanical advance takes over. I have a good seal on advance port from carburetor. It is strange that I have surging  over 50 mph now that fuel ports have been enlarged. I have a. Good understanding of how vacuum advances work. Would be a little sluggish but shoudn’t was cause a surge which I understand is a lean condition. Thanks for your help. I did discover something else as with key on I have no power to ballast resistor but car starts and runs. On chevies there is a purple wire combo
I got from starter that delivers a full 12 volts to start car but goes back to about 9 volts after ignition switch is released from start mode. Don.t understand why I have no voltage with key on yet car starts and continues to run after switch is released from star position. Guy who rebuilt carb seems very knowledgeable. Been in business for 40 years: he replaces all shaft bushing etc. thanks for reponding

James Landi

I think Daryl provided some great advice... and I suspect you'll get a new or "renewed" vacuum advance.  Since your car is used occasionally, I have had similar problems with idle and surging with a 8 cylinder boat motor.  Had a bunch of brown gunk that had plugged the idle jet, and the remedy was simply withdrawing the low speed needle adjustment and blowing compressed air through that jet opening.  The result was immediate... instead of a drip out of the idle port in the carburetor port, there was a mist -- problem solved. Same can be caused by a misadjusted float causing a drip into the intake...  Another odd occurrence with an old motor was a loose stator plate in the distributor... on this old Prestolite, the stator is attached to a bushing and as the vacuum advance moved the stator, over time a wear spot caused the stator to wobble sometimes, which, of course, caused the point's dwell to be off --- if you can move the stator horizontally with a screw driver between the distributor inside casing and the stator, you've got this as a timing issue.  CHeck out the vacuum gauge--- it's a helpful tool.   Keep us in the loop  James

Ronmcfarland

Hi guys, I pulled the distributor out and replaces points condenser rotor. I reinstalled same issue. I have a vacuum advance on it’s way. One thing I noticed is when I removed the advance the point plate moved freely. Is this right or does it mean the mechanical springs are broken. When attached to vacuum advance no movement except push firmly on advance rod. Even if springs are broken the plate is held in place I understand it affects performance but I wouldn’ think it would cause surging only at low throttle pressure. Also distributor has no side to side play. I hooked my vacuum gauge to manifold and had about 17” at idle. If it comes down to it being the carb has anyone converted over to new Edelbrock fab carburetor. Looks fairly easy with exception of linkage. Cadillac pushes and all Chevy’s I had I think they pulled. Not sure if there are places to attach linkage on bottom so it can push. Looking forward to your ideas.
Thanks
Ron

Ronmcfarland

With distributor in rotated rotor and in sprung back. Think dist is ok.

James Landi

Sounds like you're on your way to eliminating the distributor as the cause--- glad to know that you;re replacing the advance, and you;ve ruled out play in the stator bushing, and the spring load weights are working... as stated by another "helper," the stator freewheels w/o the advance driving it .  As you read the vacuum gauge, you must have detected movement off of the 17#s, or was that a steady reading?  17#s of steady vacuum "sounds" like the right specification, but do check --- on newer V8's 18 steady is the number-- it's worth checking the spec for your engine, since the surging can be a subtle gasket or fuel drip issue.  Would be odd and "telling" if there is no variance of vacuum with changes in rpm.   It's always wise to assess just what items were changed, and the carburetor rebuild seems to suggest itself again.  Have you checked for a gasket leak between the intake manifold and the carburetor. Easily done-- I keep coming back to a partially clogged idle passage ---have you tried compressed air through the adjustment needle valve ports? would be good to eliminate the carburetor before you spend good money on a new one, and then experience the same problem-- "been there, done that"  UGH...Hope this helps,  James

Ronmcfarland

Thanks James, the only thing left ignition wise is the coil and ballast resistor. I am going to replace the coil tomorrow. I checked all around the manifold and the only small leak I have is on passenger side at base of carburetor as I have had the carburetor off 5 or 6 times and have used the same gasket. I did remove the idle jets and sprayed berrymans so believe they are clear. Thanks for your help

Ronmcfarland

Hi guys, finally received a rebuilt vacuum advance. I installed it and also set the timing with a vacuum gauge. Have almost 20” of manifold vacuum. The surging is gone. Hopefully the car will run good for quite awhile as everything has been replaced. I even installed a stock rebuilt fuel pump. I learned a lot about a properly operating vacuum advance.
Thanks for everyone’s help
Ron

James Landi

SO glad you worked your way through it.  So much satisfaction when you know you've masterfully remedied an annoying problem.   Happy day,   James