carburetor Problems - HIGH RPMs

[jakecolumbia8.3]

This is going to be a lengthy description, but I have made numerous repairs/fixes and am at my wits end no longer knowing what information is pertinent or not.

Problem:
A4 starts, idles fine, when throttle is increased slightly RPMs increase all the way to up to 4000 RPMs.

Now the lengthy description. I bought a ’79 columbia 8.3…A4 did not run. carburetor was already removed. In getting the engine back in shape I’ve done the following:

1) Checked raw water flow

2) Emptied old gas from fuel tank with pump also got out a lot of sediment from bottom of fuel tank.

3) Replaced impeller (bought from Moyer)

4) Replaced thermostat with single action 160 degree thermostat and (bought from moyer). Engine was running hot at idle…problem fixed.

5) Took apart and resealed mechanical fuel pump after small fuel leak in gasket around housing of manual primer. No more leaking of fuel.

6) Replaced spark plugs. Plugs were Champion replaced with equivalent spark plugs from autolite.

7) Added 15 gallons 92 octane gas to fuel tank

8) Replaced fuel filter/ water separator on fuel line before fuel pump

9) Replaced fuel line (copper tubing running from Fuel Pump to Carb. Fuel Inlet) A4 Manual says 5/16” Outer Diameter couldn’t find 5/16’’ so I used ¼” copper tubing (1/16 smaller) with compression fittings

Now the carb.
1) Bought Moyer Marine Service and overhaul manual

2) Ran step by step through disassembly, cleaning (with Carb cleaner), and assembly in Fuel System section of Moyer manual…carb was already very clean to start with.

3) Bought Tachometer (bought from Moyer) to view what RPMs engine was running

4) Installed carb….started engine…no change. Engine idles great, a little high around 900-1000. I got it down to 800 rpms but it runs a little rough. Give it a little throttle and RPMs scream up to 4000.

5) Removed choke cable and throttle cable…operating everything now straight from levers on carb. Move throttle lever 1/8th inch throttle up RPMs jump to 4000

6) Rpms go so high tachometer breaks first day it was installed!!

7) I know what you’re thinking now…Mixture!!! Just to be sure I removed the carb and disassemble again. I TOLD YOU IT WAS GOING TO BE LONG.

We are not done yet.
8) Removed all jets, needles, gaskets, float assembly, reassembled with new gaskets (Moyer gasket kit)

9) Set all screws as follows according to a Gravely publication on Zenith 68 carbs.
Throttle stop screw - held throttle at stop position, turned throttle stop screw until it touched stop, continued screwing in 1.5 turns
Main Jet – screw in until seat turned back out 2.25 turns (left)
Idle Adjust Needle – screw in until seat turned back out 1 turn

10) Installed carb. still with no throttle cable. Same result! Idles great, starts pretty well with little to no choke (Raises questions! Addressed later.) Move throttle lever 1/8th inch throttle up engine RPMs go high. (no actual RPMs Tachometer is broken). Tried from no throttle to throttle up under load in forward. No Change! Still high RPMs with very little throttle up. Same in reverse. Unscrewed (turned left) main jet needle 1, 2, 3 full turns to lean mixture no change at all. Maybe slightly rougher idle when main jet needle out 3 full turns (5.25 full turns from seat).

11) Remove carb AGAIN. Remove flame arrestor. Plate that is the Choke Valve is bent leaving valve open even when choke lever is closed. Flatten out choke valve plate, reassembled; assured tight fit when choke is in closed position.

12) STAY WITH ME ALMOST DONE… Last thing I can think to check while I have the carb off…Float and float assembly. Took float assembly apart again, checked port, movement of main jet needle, reassembled float….

13) Checked float level height as according to Gravely Zenith 68 publication. “With bowl cover inverted viewed from free end of float the float bodies must be centered and at right angles to the machined surfaces. The float settings is measured from the machined surface (no gasket) of cover to top side of float bodies highest point” dimension should be 1 5/32” plus or minus 3/64”…my float was 1 12/32” Ah Ha THIS must be the problem. Using needle nose pliers and some pretty precise calipers I got the float centered and square at 1 6/32”

14) Carb reassemble, put back on the A4……wait for it……wait for it…..NO Change. Same problem. Idles good. Needs coke to start now. 1/8th inch throttle up RPMs go to the sky.

I’m staring to think it isn’t the carb. I need to be chasing, but don’t know what else to try any ideas at all are helpful. Don’t want to spend $150 on another tach until problem is fixed. More importantly, NO sailing at all this season because of engine trouble. I’m so close I can feel the wind. Please help.

[ndutton]

Dave Neptune will return Tuesday, he da man for these issues but in the meantime here's something to consider.

Fuel is only half the mixture formula, air is the other and the carburetor automatically supplies the proper amount of fuel for the volume of air passing through. The fact you're getting a proper fuel and air mix (engine runs) tells me you have a massive air leak but all that air is passing through the carb via normal channels. That's why the carb is cheerfully supplying a proportionate amount of fuel.

So why isn't the throttle butterfly controlling the air volume once it's cracked open? That's where I'd be looking.

[Marian Claire]

Welcome to the forum.
Just to clarify. In #10 you mention turning the main jet out, left, to lean out the mix. I believe backing the main jet out will cause a richer mix. Have you tried leaving the choke closed and then open the throttle? I had an air leak around the throttle shaft that only happened when the throttle was at a certain setting. With the carb off does the throttle shaft, arm, plate seem to work properly? No loose connections or slipping. Dan S/V Marian Claire

[hanleyclifford]

I can see a vacuum gauge in your future. Check manifold gasket, carb gaskets and pcv valve if so equipped. As Dan indicated, the adjustable main jet works opposite from the idle mixture screw. Start main jet at 2 turns off seat (counterclockwise); turning clockwise leans mixture - and idle at 1 1/2 turns off seat (counterclockwise); turning clockwise enriches mixture.

[67c&ccorv]

Jake - check the carb throttle butterfly actuating shaft where the throttle lever is clamped on.

The MMI manual states that the nut and bolt that clamp the lever to the shaft often get stripped because too much torque is applied to the tiny brass screw.

Then get yourself a Moyer Manual before doing any more work on the motor.

[CalebD]

Problem:
A4 starts, idles fine, when throttle is increased slightly RPMs increase all the way to up to 4000 RPMs.

Could any of this be a timing issue with the distributor cap etc?

[jakecolumbia8.3]

Thanks everyone for your helpful advice. I have done the following …

Marian Claire/ 67c&c -
Thanks Marian Claire for the correction on the main jet adjustment screw and which direction makes the mixture lean/rich. This made me go back and make sure my other knowledge of the screws was correct. I have reset the adjustment screws as follows on the carb.

Throttle stop – touched stop, continued turning in 1.5 turns
Main Jet – seat then backed out 2.25 turns
Idle – seat 1 turn backed out

With Carb removed from engine – checked throttle shaft, arm, butterfly for operation and slipping, everything seems to work smooth and operate ok. Check nut and bolt at the lever everything seems to be ok.

Video link of throttle plate movement:

https://picasaweb.google.com/109266274153883230898/Atomic4#5661976628185651698

I’ve heard of people while the carb is attached to the engine and engine is running spraying starter fluid around the throttle shaft to see if there is an air leak. This seems like a dangerous practice to me …has anyone tried this before??

Hanleyclifford-
Checked carb gaskets (already installed new), no pcv valve on this A4. I am not that familiar with Vacuum gauge readings, but I purchased one, hooked it up. I am still educating myself on vacuum gauge readings. From what I’ve learned so far my idle at around 16” seems spot on as far as idle goes. --- For anyone who knows a thing or two about vacuum gauges, I’ve posted 2 videos of the vacuum gauge readings I got when increasing the throttle.
Video Links:

Vid 1 -

https://picasaweb.google.com/109266274153883230898/Atomic4#5661932100032176850

Vid 2 –

https://picasaweb.google.com/109266274153883230898/Atomic4#5661932154451759906

Idle rpms = about 900
Both videos in Neutral with no load
Adjustment screws set as mentioned above

Since I need to check the manifold gasket I went ahead and removed it. Since I had the manifold off and a lot of time on my hands, the head has also been removed and valve plate. I will be completing a minor overhaul via Moyer Manual…head, valves, manifold, exhaust, timing. So won’t be able to troubleshoot until it’s all put back together. Very sooty under the head (rich mixture?)

The hope is that after the overhaul and reassembly any air leaks, timing issues, etc. may have been solved. Will post results when I’m all reassembled and firing up again.

Link of Pics – Head, Pistons, Valves, Manifold.

https://picasaweb.google.com/109266274153883230898/Atomic4

Thanks again for everyone’s input.

[Dave Neptune]

A question

Do you have a PCV valve upgrasde on the engine? If you had an air leak into the manifold it would be there all the time even at idle if not more so due to the higher vacuum at idle. I have seen a few PCV valves fail in the same manner and have seen some do so when installed backwards. If you do have one plug it off at the spacer and retry the carb.

Dave Neptune

[jakecolumbia8.3]

no PCV Valve upgrade on this engine.

[hanleyclifford]

Excellent pictures. Looks like you have a good handle on the situation. One recommendation: it appears that some coolant water may be migrating on some of your head and manifold studs - when you reassemble coat the threads with thick gooey #2 Permatex before putting them into the block. Wouldn't hurt to chase out the threads with a 3/8 - 16 tap first.

[Dave Neptune]

Ouch!

Jake, I got to a good computer this evening and played your vid's a few times. I see and heard nothing BAD. I also saw nothing in any of the parts that would cause a problem like you describe. Actually most of it looks pretty good and more than runable.
When I observed the motion of the throttle it did seem that the engine did rev nicely and maybe a bit high but nothing bad. As far as your tach I don't have any idea what happened. The sound of the engine seemed pretty normal and the idle did seem a tiny bit rough. I have a feeling that the issue is to much timing or the advance is stuck open. Either one of those will increase unlaiden acceleration but run like krap under a load. Yours seemed on the edge and that is why I say the weights may have stuck. Note they could be unstuck from disassebmly bumps but do make sure the are moving freely.
How long had it been since the timing was adjusted?
My engine looked far worse 26 years ago and it is still running after a valve job. Other than a bit of labor and some gaskets your in good shape.
Maybe not what you wanted to hear but that is my opnion after watching a few times.

Dave Neptune