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Triumph TR6 - Lousy Vacuum and idle
I am looking for suggestions in solving a vacuum problem on my 1976 TR6. It has about 10 inches vac. at 1000 rpm idle.... can not get the idle any lower. I have found no vacuum leaks and believe that the cam and crank are correctly timed. It "hunts" at this RPM and a Color Tune plug shows it to run lean dispite requiring some chock to keep it running at all. Here are the particulars:
1. the engine if freshly rebuilt as are the carbs, fuel pump, etc. All hoses have been replaced.
2. the engine has +.020 pistons with compression over 160LBS.
3. I cannot find the specs on the cam... ground by Shadbolt's of Vancouver, but know that it is roughly an S2 grind and is operating 1.55 roller rockers.
The cam timing was set by putting the crank at TDC and the cam such that the #1 I&E roller rocker tops, which are flat, were leveled with one-another while the dizzy was 180 off from firing #1 cylinder.
4. Carb floats are at 16mm, gross jets are used. Needles are set for maximum richness while jets are recessed about .100" below the bridge. I had raised them to be level with the bridge but to no advantage.
If more information is needed, I would be happy to provide it.
I am at a loss to explain why there is such poor vacuum and what I can do about it. Perhaps I have overlooked something fairly obvious? Your suggestions would be much appreciated.
Scratching my noggin, Rob
|I would check the diaphragm on the booster/|
|Hi Don, thanks for the note. Booster diaphragm is good, check valve is OK, vacuum retard dash-pot is good, gulp valve is good, anti run on valve is good. I, however, am not good.... grumpy-middleaged-greying-coot ;o(|
|Rob: I believe that a performance cam will result in a much lower vacuum at idle than a stock cam. I am running an Isky street cam with probably similar numbers to yours, and my idle vacuum is also around 10" Hg.|
Idle speed is a different matter. With the ZS, I have no trouble getting the idle down below 800 if I wasnt to. Is your ignition timing set correctly? Also, check the archives for info on leaking bypass valves affecting idle speed.
|Rob, there is a real good chance it just won't idle any lower with the cam you are using. I'm using the GP3 cam and Richard advertises an idle speed of 1,000 to 1,200 revs. My 3 carbs aren't the best and the dist. should be recurved, otherwise I might get it down to a 1,000. As far as vacuum goes, I've never had occation to check mine, but like Tom said, high perf. cams won't give you good vacuum numbers at idle because of the amount of overlap ground into the cam to allow it to run at a higher top end.|
|Thank you Rob , Tom, and Don. I really appreciate your input. I would be interested to know what the approximate correct ignition timing should be at 1000 RPM. It was suggested that I "pin" the ignition advance at max...I am not too favorable toward this and had not planned on building a race car but merely one notch up over the stock machine. Has anyone pinned their ignition? What would the recommended advance be at 1000RPM?|
Many thanks, Rob
ps. the car does rev high quickly and with significant snap having a tone that rivals any car out there.... wish I could post the sound on this site. :0)
|Rob: Assuming for the moment that you can't get the idle down to 800 or so, I'd set the static timing at 10-12* BTDC and see how it runs. Sometimes just getting the timing in the ballpark will let you get the idle where it should be.|
If you can get the idle down to around 800, you could use a timing light. Disconnect and plug the vacuum line to the retard capsule (do not plug/cap the capsule). Again, set the timing to 10-12* BTDC with the engine at idle. At this point you can either reconnect the vacuum line, in which case you should then see 4* ATDC (ATDC, not BTDC), or leave the line plugged permanently as many of us have done, and take it for a drive and check for pinking, throttle response, etc.
Let us know how it works out.
BTW, are you sure there's no binding in the throttle linkage or couplings that would prevent either carb from closing completely? Perhaps try backing off the idle speed screws and even loosen the inter-carb spring connectors, and make sure that both carbs close completely.
|Tom made a good point about checking the carb linkage, and have you checked that the carbs are balanced? With the cam I have, I run the dist. at 16* at idle speed.|
|Do all the members of this forum know of Ratco linkage??|
A must have!!!
|Rob , Tom and Don, again thank you for the input. I will try a 16 degree advance with a disconnected retard. Linkage is good: smooth , lubricated, rebuilt, replated. Thottle plates close perfectly: allignment is set with no binding. Throttle shafts and bushings are perfect: no wear, smooth, replaced seals. Carbs are balanced using a uni-syn tool.|
Tell me more about the Ratco linkage Don.
I will work on it this weekend, I hope, and post what the results are.
|Rob- If you ever do a fine check on the stock linkage you will notice that it doesn't allow for a complete throttle opening. This will allow this and smoother.|
This thread was discussed between 26/11/2008 and 28/11/2008
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