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View Full Version : TR5/TR250 TR250 "gives a little cough" at times



RobT
04-27-2006, 09:10 AM
So I've been enjoying the nice weather we have been having in Eastern Mass the last couple of days and putting a few miles on the TR.

Its going really well, the suspension upgrades over the winter make it even more fun to drive.

The only problem is that, under load at low revs (<2k) it gives a little "cough" and splutter, hesitates and then once I am above 2K revs, of it goes.

Does not feel like ignition (but could be), and the carbs (triple ZSs)are recently re-built, so not sure what is going on.

Here is my set-up:
Point-less electronic ignition
Triple ZSs with auto transmission fluid in the dash pots (as recommended by Joe Curto, who rebult them)
Goodparts "mild" cam
Stock distributor, with both vacuume retard and advance disconnected
Timing at 10deg BTDC
Compression between 180-190 lb on all cylinders

I have a couple of suspisions:
1) I need to reconnect the vaccume advance on the dizzy
2) Different oil in the carb dash-pots (lighter or heaver?)
3) Vacuume leak

Or it could be something simple like dirty plugs.

So can antone help me out with any suggestions they may have before I start in taking things apart etc?

Thanks in advance for the advice.

Rob.

Andy Blackley
04-27-2006, 11:03 AM
Rob: Try the heavier oil first. ATF is too thin IMHO. I use motor oil.
Consider reattaching the vaccuum advance (only).
Do you know which metering needle you are using in the carbs?
BTW which plugs are you using?

RobT
04-27-2006, 11:32 AM
Thanks Andy - I will hook-up the vacuume advance. This was going to be my first step.

Any thoughts on the grade of motor oil to use? Is 20W50 too heavy? Perhaps 10W30?

Not sure what the needls are - is there any way to tell?

As for plugs - I am using the Bosch Platinum plugs from VB.

Thanks again,

Rob.

Andy Blackley
04-27-2006, 11:44 AM
I use 20W50, especially in the summer. ATF is OK in the deep winter, but I havent driven an LBC through the **** of a nortnern Ohio winter since 1990. Now I drive a BBT.
Which carbs are you using? The only way to check the needle is to remove one and look a for the markings on the shank.
I use good old Champion UN-12Ys.

04-27-2006, 11:54 AM
Hey Rob,
Darn if I don't experience the same exact thing with my car.
We have similar setups. The only real difference is I am running basically stock ignition although I do have the hotter Sport coil and fat low resistance plug wires with cold Champions. I will get a little cough or burp at low highway speeds, with a very mild throttle. I have checked every possible option for correcting this, save reconnecting my vacumn advance. I don't really see what this could do. I am beginning to wonder if this isn't a little quirk of the trips setup. It seems to go away after some time, perhaps after half an hour or so of cruising. It can't be a fuel problem as it seems to come and go with such quickness. There is way too much fuel in the system for such a burp to occur. It almost seems electrical in nature, or timing. I am at a loss as to what the issue could be.


Bill

bobh
04-27-2006, 12:32 PM
Take a look at the PDF on the site listed below. Even though it is based on Volvos there is some very good information. Particularly the section on acceleration on page 2-39. Note the mention of ATF
My guess is that when you hit the gas pedal to accelerate you have a momentary rich condition. The engine can't quite handle the extra gas from the three carb setup. One or two hiccups and the engine starts to get enough air to digest all of the gas.
I've experienced a similar situation on a motor cycle with a carb and an accelerator pump. I set the pump to it's richest setting. Below 3500 rpm the bike would cough due to the extra squirt of fuel then take off like a missle. Above 3500 it digested the gas immediately and took off. Granted the engine design and speeds are different that the TR, but the symptoms sound familiar.
One of the Healey guys, GregW , put 3 Mikunis on his 3000. They have accelerator pumps. Although they are different than Strombergs, the momentary rich condition may be a common issue. He said it took a while to sort the carbs. He may have some ideas.
https://www.dctra.org/files/carb.pdf

DougF
04-27-2006, 02:19 PM
I don't think it would be the oil. I'm not running your configuration, but do run Marvel Mystery Oil in the dashpots with no problems.

vettedog72
04-27-2006, 02:46 PM
Reconnect the vacuum line to the distributor and part of (if not all) of your problem should go away. The mecanical advance is for WOT; you can benefit from the vacumm advance system with anything less than WOT.

Andy Blackley
04-27-2006, 08:25 PM
I think the problem is a momentary too lean condition. If the carb piston moves upward too rapidly the carb actually leans out until RPMs increase.
A simple engineering formula for flow is Q=V*A. Q= flow rate, cubic feet per min., a function of engine RPM; V= velocity, A= the cross sectional area of the carb opening.
At a rapid throttle opening the piston rises, and if undamped by oil or a stiffer spring, the cross sectional area increases faster than the RPM. Therefore the velocity will decrease until the RPMs increase. The loss of velocity results in less negative pressure over the jet bridge (venturi effect), which means less fuel is drawn upwards through the jet, even though the jet opening is larger.
This momentarily results in a lean condition, hence the stumble.
Having three carbs doesn't matter, as the carbs operate in proportion to the flow rate.
Try a simple experiment. Next time you feel the hesitation, try repeating the condition again with the choke pulled out slightly to enrichen the mixture. If the hestitation goes you found the problem.
The cure ranges from simply using heavier oil in the dashpot; a stiffer spring (red)in the air valve (piston); or a richer metering needle.
Heres a link to one of a jillion such articles: https://www.jetlink.net/~okayfine/su/damper.html

RobT
04-28-2006, 10:33 AM
Thanks guys for all the great advice.

Last night I reconnected the vacuume advance and replaced the auto trans oil in the dash pots with 20W50 motor oil.

Another glourious morning today, so drove the TR to work (the most direct route from home to the office is about 3 miles, but on a nice day in the TR, I can do it in 15 miles!)

Well the car is much happier with the changes, still a little stumble from a stop, but not nearly what it was. While I was working on the carbs last night, I also leaned out the mixture a tad, which given what Andy says above may have been a mistake. Also noticed a little pinging too. So I'll bring the mixture back up again and see how she runs.

Once again, thanks for all your help!

vettedog72
04-28-2006, 06:38 PM
Rob:
If you have a bit of ping well off of max RPM, I would turn the distributor to retard the timing a minute amount at the time until there is no ping. If you want the highest performance from the engine, you can adjust the timing more aggressively.

RobT
04-28-2006, 07:30 PM
Thanks "vettedog". I'll give that a try too.

BTW - What is the best way to adjust mixture?

Perhaps I should start a different thread on this issue, as I'm sure there are many an opinion.

I've always done it pretty much by trial-and-error. I have a Gunst colour-tune, but never had much sucess with it, and with a triple-carb set-up, doing it by ear is pretty much darn near impossible. Any favorate "tricks" folks have?

Rob.