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Carb Issues 2.8 V6

SeanP

Donation Time
So I have been having some rough idle issues and the car has not wanted to idle after driving when coming off throttle. I have checked float levels replaced the carb/manifold gasket and tried to adjust the air fuel mixture but have not had any success.
I have had a couple of gurus look at it and both commented that they felt that it wanted more air fuel but we are running out of screw to turn at 3.5 turns.

My engine was bored 30 over, has a moderate cam and had the heads ported and over sized valves (i believe exhaust only), headers, and is currently running a 390 cfm 4160 holley. I was told that I prob needed to up my primary jet size by 4 or 5 and just wanted to see what you guys are running. The 4160 came stock with a 51 currently it has 57's. Does that make sense for the jets to be 10-11 sizes over the suggested jet size for the stock motor?
thanks

also what vacuum pressure should i see at idle? Is there a good service manual for this motor out there?
 
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RootesRacer

Donation Time
Assuming the fuel level and other unrelated issues are not the issue, any time the engine wants to run with the idle mixture screw too far one way or the other it signifies that the idle jet is not correct and needs changed.

Sounds like you may need a larger idle jet (not main jet).
 

V6 JOSE

Donation Time
Try replacing the power valve. The engine may have backfired somewhere along the way and blown it.

Jose
 

SeanP

Donation Time
I have a rebuild kit but it does not seem to be a rich situation the idle keeps responding well to more idle mixture screw but at 3 and 1/2 turns its starting to run out of adjustment. Just curious Jose what carb set ups do you usually run what jet sizes?
 

RootesRacer

Donation Time
I have a rebuild kit but it does not seem to be a rich situation the idle keeps responding well to more idle mixture screw but at 3 and 1/2 turns its starting to run out of adjustment.

Doesnt sound like its rich, if the idle mixture is lean, screwing the mixture adjustment out makes it richer. If thats what the engine wants, then you are presently lean and out of adjustment range of the mixture screw.

If in fact there are no other problems with the carb, one would go up a few jets on the idle jet. If after going up with the idle jet, the progression is then weak/lean, it means you had a vacuum/air leak that you tried to compensate for with more screw.
 

V6 JOSE

Donation Time
Hi Sean,

I used 54s all the way around as a starting point. I recommend having it dynoed later to fine tune to your altitude.

Jose


I have a rebuild kit but it does not seem to be a rich situation the idle keeps responding well to more idle mixture screw but at 3 and 1/2 turns its starting to run out of adjustment. Just curious Jose what carb set ups do you usually run what jet sizes?
 

crs

Gold Level Sponsor
"So I have been having some rough idle issues and the car has not wanted to idle after driving when coming off throttle."

Sean,
My V6 65 Alpine has the same carb as yours on a 3/4 race engine and when I first began driving it, it did exactly what yours did as you state in the quote above. Most of the time when driving at road speed and then coasting toward a turn or red light, it just died.

Much research and thought went for naught, but once when tinkering with the air intake cap(plenum) removed, I noticed that sometimes the carb air intake plate would close and not reopen easily. Why? There was a fine casting flaw on the inside of the air passage and the plate seemed to be hanging on that. Judicious use of a fine metal file smoothed this out so that it was no longer detectable. It tested good and I put the input cap back on and drove it - all OK and it is still OK after several months of driving.

Whether or not this makes sense, it is the only thing that I did and the problem went away. :) Maybe the darn thing just "broke in" and my filing did nothing to correct the problem; who knows?
 

daniel luna

Donation Time
carb issues

some thing like that happen to my car a few months back, but the carb had been rebuild and tested. so my engine would stop running when coming to a stop light or slowing down so, I later my ignition moudle gave away and put
new one on since then no more carb problems I think you should check the ignition module. just my 2 Cents
 

SeanP

Donation Time
thanks for the suggestions guys I checked the power valve and it is holding vacuum so should be fine. There does seem to be a lot of play in the secondary throttle shaft and spraying carb cleaner there at idle is causing the idle to increase of get smoother.

What vacuum are these engines pulling at idle with a moderate cam? this could help me determine whether it is a vacuum leak I am dealing with. I have a new Mallory electronic distributor on order and will go through the ignition once it arrives but currently I'm running a Mallory Dual points distributor so there is no module.

Roots Racer
When you say idle jets are you speaking of the emulsion jets? At least for the 4160 there does not seem to be an adjustment there.
 

RootesRacer

Donation Time
Roots Racer
When you say idle jets are you speaking of the emulsion jets? At least for the 4160 there does not seem to be an adjustment there.

Yes it appears you are right.
Holley put a fixed idle fuel restrictor (jet) and a fixed air bleed for the idle circuit. They are pressed into the carb case and are supposedly what ALL engines need.

I dont think that's a great idea, but it is what it is and it appears that if the mixture is at its limit (out) then you probably have a vacuum leak you are fighting.

Is this a used or new carb?
 

SeanP

Donation Time
I believe this carb was installed new about 30,000 miles (8 years) ago when the transplant happened however I cant be sure since I did not do the work.
 

gordonra

Donation Time
Just got my performance mods in and I'm getting about 10 in/Hg at idle. Idle mix screws are out about two turns. I've got Jose's spec for regrind of a stock cam.

10 inches seems low, but it runs pretty well.
 

pcmenten

Donation Time
I had a couple of thoughts. Have you looked at the plugs to see if they are all the same color? Has the engine been using oil? If the intake manifold gasket had a leak in its underside, it might be hard to spot. Looking at the plugs might help identify where the leak is.

Also, do you have power brakes? I have an MGB and had trouble tuning it. I finally became suspicious of the brake booster. I took the booster apart and found that the diaphragm rubber had hardened and cracked, and was leaking air.
 

SeanP

Donation Time
I have no booster. I have been waiting to pull the plugs until I have some replacements. If I'm going to pull them might as well put some new ones in.

As far as the plugs telling me whether I have a manifold leak, what exactly would I be looking for? white plug due to lean condition near the leak?

I'm installing a new Mallory Unilite Distributor and was hoping you guys could help me ferret out the wiring. Unfortunately the last owner rewired the car with a Kwikwire wire harness and did not keep any notes on the color codes. Here is a pic of what is currently installed and what I'm planning on doing.

The way he had it wired before seems odd or unlike any example diagrams I can find. Seems to me he was only protecting the points and not the coil.
 

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RootesRacer

Donation Time
I have no booster. I have been waiting to pull the plugs until I have some replacements. If I'm going to pull them might as well put some new ones in.

As far as the plugs telling me whether I have a manifold leak, what exactly would I be looking for? white plug due to lean condition near the leak?

I'm installing a new Mallory Unilite Distributor and was hoping you guys could help me ferret out the wiring. Unfortunately the last owner rewired the car with a Kwikwire wire harness and did not keep any notes on the color codes. Here is a pic of what is currently installed and what I'm planning on doing.

The way he had it wired before seems odd or unlike any example diagrams I can find. Seems to me he was only protecting the points and not the coil.

Niether look right to me.

The ballast should go between the ignition sw power and the + side of the coil (yes it matters as the secondary returns through the primary coil).

Unless you have an after market tach (not a stock tach) there will not be a tach wire at all as the tach runs off impulses from the ignition switch feed to the ballast/coil+.
That is, the ignition switch goes into the tachs internal transformer, comes out of the firewall to the ballast resistor. The other end of the ballast resistor goes to to coil+, then the coil- goes to the ignition box/unit. The box/unit will also have a +12v power lead and a ground lead.
It may bee needed to bring in a "clean" +12v power lead instead of the one coming off the tachs transformer output lead as the current impulses from the box/unit may interfere with the stock tack. It would be OK to try it our with "dirty" power to the box/unit to see if the tach functions OK.
 

SeanP

Donation Time
I have all after market VDO gauges. I'm assuming its the Tach what else would be connected to the neg side of coil from the firewall?

I think the proposed diagram has the ballast between the + side of coil and the ignition doesn't it?



Niether look right to me.

The ballast should go between the ignition sw power and the + side of the coil (yes it matters as the secondary returns through the primary coil).

Unless you have an after market tach (not a stock tach) there will not be a tach wire at all as the tach runs off impulses from the ignition switch feed to the ballast/coil+.
That is, the ignition switch goes into the tachs internal transformer, comes out of the firewall to the ballast resistor. The other end of the ballast resistor goes to to coil+, then the coil- goes to the ignition box/unit. The box/unit will also have a +12v power lead and a ground lead.
It may bee needed to bring in a "clean" +12v power lead instead of the one coming off the tachs transformer output lead as the current impulses from the box/unit may interfere with the stock tack. It would be OK to try it our with "dirty" power to the box/unit to see if the tach functions OK.
 

RootesRacer

Donation Time
I have all after market VDO gauges. I'm assuming its the Tach what else would be connected to the neg side of coil from the firewall?

I think the proposed diagram has the ballast between the + side of coil and the ignition doesn't it?

Yeah you are right. The picture is small (looked to me like the ballast was still on coil-), the proposed should work.
 

pcmenten

Donation Time
Regarding the plugs, if you have a leaking intake manifold gasket under one of the intake ports, you might find you have five whitish plugs and one plug that looks like it has been oiled and might be darker.

Good luck.
 

SeanP

Donation Time
Well got the new distributor and coil in last night. The restart went almost without a hitch. I think I got is stabbed one tooth off and had to turn the distributor counter clockwise as far as it would go to get started. Will attempt to re-stab it tonight. It was encouraging at first, idling strong after warm up (180deg). I adjusted the idle screw to about 850 and tweaked the mixture screws until I was able to get about 15-16 vacuum steady 1.75 turns on mix screws. Took it for a drive around the block and it seemed a little sluggish but as soon as I got off throttle after jumping on it for a few blocks it stalled off throttle and would not idle. Got it back in the garage and it refused to run with out my foot on the gas. I had re-installed the 57 main jets because 62 just seemed to big and when I bought the car a few weeks back it seemed to be running fine with the 57's so I may need to adjust the floats again but I doubt its causing the idle problem. Just don't have enough trouble shooting experience to know what to look at. What might be causing it to not idle after a little hard driving?
 
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