One carb not drawing-what's up?

[jonalley]

Hi All, I'm at a loss. I have one carb ( off side front)of an IDA set up drawing on my syncrom' at a very different level. 2-3 vs. 12-14 on other three.Cylinders 1&2 do not appear to be firing even though a timing light indicates a pulse. I've done the following-
1. leak down test on cyl 1&2 -o.k.
2. took top off carb/jets and adj screw out and blew out carb
3. float appears to be working properly
4. made sure all holes appeared open in idle jets
5. have turned the adjustment screw from an initial setting of 3/4 out to 1/2 out and in. No difference although in did seem to drop the idle a bit.
When I blip the throttle the carb draws but not at idle.
Other carbs appear to be o.k.

Any ideas about where to look next?? regards, Jon

[dallas_]

Did you check the plugs on those cylinders? Sounds like you are getting a spark, but check the wires, distributor and pull those plugs to confirm they are firing.

[Gaz64]

If you rev it to 3000 rpm, does it have close to the same airflow as the other cylinders?

Leakdown test by itself is a good secondary test after a compression test.

A misfiring cylinder will not alter it's own airflow reading, it will affect another cylinder in the firing order.

Example: one could have a broken inlet rocker arm, leakdown of 3%, and rough running due to no intake stroke.

Swap 2 carbs around, you may have a twisted throttle shaft.

[23TGW]

I had that problem too.

If you run the idle screw in to raise the rpm does it draw? What's your idle rpm?

My problem was due to not having all my carbs synced. I synced them and that eliminated the problem for me.

[jonalley]

Thanks for the input. I swapped out plugs on 1&2 plus wires with some spares I had and still no results. The draw does come up to match the other carbs on a throttle blip.
If you remember an old post I made I was frightened of cylinder wash in this new build but having the draw jump would suggest the two cylinders are o.k. wouldn't it. I'm trying to count out cylinder wash but will do a compression test to check.
I'm trying to get to the syncing stage but have the very low number draw on the front carb to adjust against. Should I try to bring the otherer three down to that number??
Swapping carbs around is another step. I was trying to not take my linkages apart. Thanks, jon

[dallas_]

Can you post a picture of your linkage setup?
Let's see how you are making the adjustments for your airflow.

[jonalley]

Well I think we found the problem. I have 160-180 pounds in cylinders 3-8 and 65 in 1&2. I say not good. O.k., so I'm grabbing at straws here but since this is a new build do you think those rings might finally seat or am I looking at tearing the whole thing apart. Anyone got a really good epoxy??? Jon

[Barnsnake]

I would run another compression test with the throttles wide open. If the throttle plates for those cylinders are completely closed it will cause your idle problem and skew your compression readings.

[Rick Parker]

Swap 2 carbs around, you may have a twisted throttle shaft.

This is more than likely the issue. What happens is that one throttle blade is open slightly while the other is completely closed. You measure the one that's open and assume somthing is wrong with the other(?) it can't pull air it's closed. Try this to validate: Disconnect the link beween the two carbs on the bank that is flawed. Start the engine and then slightly increase the throttle opening on the carb in question, see if the carb comes on line and the engine starts running on all 8. If so you have a slightly twisted shaft. Use small 4 or 5 mm (??) on each end of the shaft to correct the twist, be carefull do it gently, this is one area you dont want to exert too much force.

[jhv48]

Are you the guy that had the problem with valves that weren't closing completely?

With that compression figure I would do a leak down test on the two low cylinders and see where the air is leaking.

[Gaz64]

Jon,

You stated in your previous thread that your tight valves were the cause of your low compression and high leakdown.

On your Ford V8, cylinders 1 & 2 are side by side and fed by the same carburettor.

Did you have the throttle wide open permanently while you compression tested all 8 cylinders?

If you did, and now still have 65 in 1 & 2 and 160-180 in the rest, I'd be doing another leakdown test. BUT if the leakdown doesn't show your loss, you could have a split bore between cylinders.

If you didn't have the throttle wide open when you did the compression test, then if the affected cylinders were trying to draw through a almost closed carb, you get low numbers.

It is highly unlikely to have low even compression on adjacent cylinders due to unseated new rings.

[zrayr]

Originally Posted by jonalley

"..........I'm trying to get to the syncing stage but have the very low number draw on the front carb to adjust against. Should I try to bring the others three down to that number??......."

reading on I see you have a compression issue, but for healthy engines it doesn't take much adjustment to bring up the vacuum readings. A quarter screw turn can be the difference between zero and "all balanced".

Z.

[Gaz64]

Originally Posted by jonalley

I'm trying to get to the syncing stage but have the very low number draw on the front carb to adjust against. Should I try to bring the other three down to that number?
Thanks, jon

Jon,

You should be able to raise or lower the airflow of any of the carbs independent of the other 3.

If you can do this with one of the other carbs and not with the "NO. 1" carb, then your 65 psi in the two cylinders is still your main problem.

[jonalley]

Just reporting back in. I realized that the compression tests were not done with WOT. Redoing those produced good numbers. From everyone's comments I also realized ( and correct me if I'm wrong) that a totally closed throttle plate is not acceptable- slightly open is in order and I moved the linkage adjustment to set that in place. I have had a measure of backfiring too and noticed that two of the carbs show a somewhat wet area at the gasket from the carb to the manifold. I think there is leaking. I am going to pull the carbs off and use some gasket compound on those intersections. I'll report back in as soon as I get a chance to do the work. Thanks for all your support. Jon

[Rick Parker]

Stop! Do not put any sealant on the gaskets. They are leaking at the gasket because the floats are sett to high or the needle & seat is not closing. This allows fuel to leak out of the auxiliary Venturi onto the throttle plate where it pools and then soaks the gasket. No sealant!!!

[Eljaro]

I would take the valve cover off and look a the valves and the rockers. If the leakdown test is ok but the compression is low you may have those valves not set right.
When you check compression you should have the carbs throttle plates wide open, otherwise you will not draw enough air into the cylinders.
You can also be having a bad seated intake gasket and the engine is drawing air through there.
You could also have you linkage not set right. Check that all carbs have the idle position set by the linkage and not by the individual stop on each carb.

[Eljaro]

I missed a few posts and see that the mismatch on the compression was effectively the not WOT position of the carbs.
If you have fuel around the base it is because the fuel dumped into the throats is not drawn into the cylinders because of the missing vacuum and stay on the throtle plates dripping down slowly and wetting the base gasket.

The manifold intake gaskets,especially the ones from felpro with the print-o-seal bead move when you tighten them down, and often slip out of place allowing air to be drawn from there. There might be no gasket now between cyl 1 and 2 and both cylinders may be drawing air from each others intake runner.

[jonalley]

Rick- I'm searching for solutions to two problems-I think( carb draw and backfiring. The wet look to the gaskets under the front two carbs suggest leakage which makes your comments on target. I tried tightening down on the allen head bolts but they are as tight as I can get them.
Eljaro- Yes, I think the compression is o.k. I have not run the engine since as we are getting rain here and I don't want to start the car in the garage. I'm more confident that the carb will draw now as I opened the throttle plate a bit with the linkage. The wet look to the gaskets of the front two carbs suggest that it could be causing the backfiring ( also,the backfiring settles down as the engine warms- parts expanding and closing gaps??). When you speak of gaskets I assume you mean the individual gaskets under each carb and not the gasket unter the whole intake manifold. I have no idea what brand that is if it is the case.
As soon as I can I'm going to run the car and try to determine what steps to take next based upon what I've learned from you folks. Thanks so much. You are a real help. Jon

[Eljaro]

I mean the gasket between the intake manifold and the heads,the one which needs the manifold to be removed in order to get at it.

[jonalley]

Eljaro, That's a serious amount of dismantling. Is there any way I can test for such leaks-e.g. the way you use soap suds on a gas pipe assembly. I know it's suction not pressure but ???
I've gone over all the carb and maniflod bolts to test if any were loose. All seemed fairly tight. If I can I will start the car tomorrow( Monday here) and see if it runs the same. Then I suppose I have no other recourse but to start taking things apart. Thanks all. Jon