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Need help diagnosing fuel mix problem

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ADW

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The mix in cylinders 4,5,6 is always rich - 6 more so than 5 and 5 more so than 4. I need help further diagnosing - thanks in advance.

What I've found so far:

1. Cylinder 6 plug dies after about a month and shows heavy black deposits. 5 and 4 are alos dirty but not as much.
2. There is slightly lower compression (10%ish) in cylinders 4,5,6, than in the others.
3. I installed new DGV carbs recently.

Could this be a vacuum leak? How do I diagnose this further?

I thought there might be a valve problem in cylinder 6 but why would cylinders 5 and 4 also be affected?

There are no other signs of a head gasket leak but is that a possibility?

I could swap the carbs to make sure this is not a carb needle problem but that seems unlikely given how new they are?
 
Your profile says you have a TR6. I am not familiar with DGV carbs. Do they have a mixture screw like the carbs that came with the TR6 so you could try to reduce the fuel flow on the rear carb?
 
Thanks for your interest Phil. I should have given more detail. The Weber DGVs have a mixture screw that is the primary way of adjusting the mix from factory settings. I'm using a Gunson Colortune to refine the mix and cannot get anything but a rich mixture in the rear carbs, no matter how I adjust the mix. I had the same problem with the stock Strombergs hence I'm less inclined to believe this is a carb problem.
 
A vacuum leak would cause a lean mixture.
 
Have you checked valve lash, and lift, on 4,5,6? Sometimes valve problems can mess up the mixture by causing reversion. That might also explain your lower compression readings.
 
poolboy said:
What you driving, neighbor ?
Click to expand...
I was just curious, sorry about that, since I live just on the other side of Tylertown....
 
Good procedural document, Ron!

ADW: When you do this, are the linkages connected? Reason I ask is I think it possible the linkage between the three carbs isn't correct, leading to the carbs reacting in "sequence" rather than simultaneously. Back carb being the richest, front being leanest is why I'm suggesting this.

I'd be for getting it up to temp, shut down and disconnecting the linkages before tweaking the things. Start it and adjust. After they're set check/adjust the linkage to have the same amount of movement of the throttle shafts from front to back.
 
Doc, THREE carbs???

I thought I read DGV's, not DCOE's... It would be an interesting application if he got three DGV's on an inline TRiumph engine..

Course you're quite correct in that the linkage definitely should be disconnected for final idle speed setup.
 
I hadn't thought of checking that. Thanks.

Would a leak in the head gasket cause an overly rich or lean mixture in the affected cylinders?
 
If you get a chance check your messages, up there in "MY STUFF".
 
I checked and adjusted the clearances and tried to adjust the idle mix with the linkages disconnected....same problem no matter how I adjust the idle mix on either carb 123 ok, 456 rich to richer. Should I swap the carbs next to make sure this is not a carb problem or ....? Thanks in advance.
 
ADW: Swappin' 'em would likely give you more data. If the result changes, the richer carb may have cracked idle mix needle seat cracks or different idle jets. If the condition exists as before, look to linkage issues. Did you visually check to insure both carbs had the same mechanical shaft movement in synch?

Ron: another "assumption": I recall seeing a triple DFV/DGV setup on a TR-6 and made the leap. Pro'lly not a "common" setup.
 
Will do. Just so I'm clear, how would a linkage issue affect my not being able to set the right mixture at idle?
 
If the link between the two has any tension on it you don't have both carbs' throttle plates in the same position relative to one-another.

But also, the: "not being able to set the right mixture at idle" could be the result of someone over-tightening the idle screw (and cracking the seat in the casting) on the one you can't get to settle down. Disconnecting the linkage unfortunately just rules out the former. A crack in the seat of the idle adjuster screw is not easily repairable.

Since these are new units, I'm more inclined to believe the throttle plates are in different positions and you may be seeing that rear carb picking up fuel due to either linkage mis-adjusted or throttle stop variance.

With the colour-tune in #5 and linkage disconnected, have you tried backing the throttle stop screw out some on the rear one? THEN using the mixture screw to make a change?
 
Dawns on me: Which choke arrangement is on these things?!?

And jump in here any time, Ron (or any of the rest of yins)! We mustn't let this beat us. :laugh:
 
The DGVs have manual chokes. Your post made me go check if both chokes were moving (and returning) freely. I'm not convinced there is enough return tension in the problem area carb. I've made some changes (below) with bizarre (to me) results - maybe you can solve the puzzle with this info:

1. Physically made sure closed chokes were fully disengaged.
2. Disconnected accelerator linkages to carbs
3. Set Idle mix on both to recommended starting position (1 turn out from full rich)
4. Set #2 to blue on idle with a little yellow at high revs
5. Set #5 to almost blue on idle
6. Reconnected linkages and checked synch - all ok at various revs

Here's what's weird. I stumbled on the setting for #5 since a 1/6 turn of the idle screw takes you from rich to almost correct back to rich ---- while leaning out the mixture! What's up with that? Something else must be coming in to play.
Any ideas? Thanks again for your help.
 
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