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Triumph TR6 - 'Sneeze' at high rpm

I have miss, or more lika a sneeze at high rpm (~4000+) when accelerating, its not really bad, but very annoying... Just when you really get going theres this "sneeze", i guess it just misses, theres no popping or back-firing.

I really dont know if its electrical or fuel..

I have a new sports coil, new HT leads, new plugs, newtronic ignition. I run on SU HS6 carbs, K&N's and a new electric fuel pump. (theres also a TT road83 camshaft, headers, a "PI-head", but i dont think they are responsible for this.)

I usually have the timing set at 10-12 btdc. I have tried to "read the plugs", and as far as i can tell they look as they should, but i'm not sure, and it is a bit hard to read them right after accelerating, thers allways a bit of slow driving in between.

Except for this sneezing it runs lika a champ, and as far as i know the electrical side should be ok, its basically new all new from coil to plugs, distributor is working as it should (advance etc). So i am suspecting a lean condition, and the next step is to install an o2 sensor.

Any ideas? Should i start experimenting with the dashpot oil? Pull the choke when accelerating? Still something to check on the ignition side??




Hi Mat,
The dashpot oil only is effective when pulling away from idle, at high rpm's the piston is typically opened up to allow fuel / air in and at that point the dashpot oil is "not working"
I had a similar problem when I slowed down to take a corner and downshifted..mine stumbled or missed at the point of turned out to be the plate below my led sensor points for the electronic ignition. At that certain point of torque the led and disc would go out of alignment causing a miss. There is a tension clip on it that had broken off and it wobbled..only took 2 years to figure that one out !
May be worth a look.

Charlie B.

How about the fuel filter? Maybe slightly plugged and starves the carbs a little at higher rpms. Possible even if you have a new one since our tanks can be bad for rusting. Just a thought.

Doug C.
Doug Campbell


Your Dashpot Oil comment begs the question and I apologize for no doubt boring many....What is the proper weight to use? I have read many differing opinions. Thanks!
Jeff Shirhall

Hi Jeff,
The Bentley book states to use "seasonal grade oil", Haynes suggests engine oil...I use 10w30, but I also tried automatic transmission fluid and even 90 wt gear oil figuring oil will thin out as the carbs get warm and I must say I never noticed any real difference.
Charlie B.

Mat: What metering needle are you using in the SUs? Might be a tad lean at the upper end.
A Blackley

Thanks for all the comments and suggestions so far!

Charlie, thanks for the tip.... And youre right of course regarding the dashpot oil!

The fuel filter would of course be an easy way out, definitely worth checking, allthough its only 1 year old and I seem to remember that the problem was there before & after I changed it.

This needlebusiness is getting me more and more confused the more I try to study it... (hence the o2 sensor!).

I think I have BAM needles, wich are according to moss-europe the richest needle suggested for this level of modification (they have a quite good website with downloadable pdf's on modifications etc). However, according to Burlen the suggested standard needle for HS6 on a TR6 is BAG, wich is more or less the same as BAM...

So I think the next steps for me will be:

1. Install the lamda sensor
2. Check for air-leaks in carbs, mainfold etc
3. Depending on the findings with the o2 sensor take it from there.

Question: is there an easy way to measure the fuel pressure to the carbs? In what range should it be?


Matt--Where did you source your HT leads? Check your dist cap for tracking.
Rick Orthen

Hi Mat,
You may want to check your spark plugs too- I have had similar problem in the past (with Champion plugs on two occasions!) I suggest replacing them all at once if that doesnt cure the problem no worries you have an extra set of plugs which you are going to use anyway.

Matt: The fuel pressure for SUs should be in the 2 to 2 1/2 PSI range I believe. More than 3 PSI is too much. If you are using an SU elec. fuel pump no problem, as it is regulated to deliver this pressure. If you are using something aftermarket like a Holley you will want to install a pressure regulator.
Here is a link to a TR250 site that discusses SU tuning with an o2 sensor:
A Blackley

PS You can install a simple fuel pressure gauge tee'd into the line just ahead of the carbs just to check the pressure and then remove it.
A Blackley

This thread was discussed between 13/03/2006 and 15/03/2006

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