Help with engine problem

[TFR]

I have my TR3A running well except for a problem with my carburation (sp? Too tired to look it up)
When the engine is at low RPM (1500 to 2000) and under load it back fires. My understanding about back firing is that it is a lean condition. Is this correct?
My distributor is electronic (Mallory) and I have the timing set at 4 deg BDC. It seems to run great. So I don't think it is a firing problem.
Any ideas would be appreciated.

Also how do you guys set synch the carbs?

 

[DrEntropy]

Ummm... "Backfiring" is a bit less than descriptive of the symptom, Larry. Through the carb or the exhaust? A report from the carb would mean lean, exhaust would mean rich. Not trying to be a smart-butt, just looking for a bit more definitive info! I'd also say the dizzy timing "lead" is a bit less than it might be. Say a better setting would be around 5~8 degrees?

Are you setting it static or dynamically?

 

[TFR]

It is back firing through the carb.
Also, I use a timing light. I may need to adjust the auto advance on the distributor.

 

[tomshobby]

Mine was backfiring and of the several problems found when it was torn down there were two that could have contributed to backfiring. One was a hole in the distributor retard diaphragm and the second was the three worn cam lobes.

I believe the cam problem was the biggest contributor because there would have been three valves with potentially different timing than the rest.

 

[Geo Hahn]

Also how do you guys set synch the carbs?

I use a Uni-Syn tool. Given the size of the balance tube on the TR3A manifold I don't think this is a super critical setting so the Uni-Syn seems to me to be adequate.

 

[TR3driver]

Also how do you guys set synch the carbs?

I use the "SU tool kit". Cheaper and more rugged than a Uni-syn, plus I like being able to check the synchronization with the throttle slightly open as well as with it closed.

The TR3 linkage deflects and allows the rear carb to lag a bit if you do it "by the book", so I first adjust the linkage to get the carbs in sync with the throttle held open a bit, then use the idle screws to sync at idle. In theory, this should make the engine a little smoother off-idle, although in reality I can't tell the difference /bcforum/images/%%GRAEMLIN_URL%%/smile.gif

The SU kit will also show you if the air pistons are moving together when you open the throttle. If they don't, there is something wrong.

 

[Geo Hahn]

Randall --

When you use the SU kit do you leave the air cleaners in place or remove them?

 

[MGTF1250Dave]

Aloha Geo,

Like Randall I use the SU tool kit. When I so this I leave the air filters in place.

 

[TR3driver]

When you use the SU kit do you leave the air cleaners in place or remove them?

Whichever is convenient at the time. Usually, I'm adjusting the carbs as part of a tune-up, which means I'm changing the filters (I use TR4/A paper filters instead of the stock "rock catchers") anyway. They should have no effect on carb sync.

 

[martx-5]

It is back firing through the carb.
Also, I use a timing light. I may need to adjust the auto advance on the distributor.

The 4 deg BTDC specification is for a static timing adjustment, not while it's running. I don't know if the Mallory distributor has enough advance at idle to make a differnece, but the original Lucas does.

 

[TFR]

Art, I built motorcycle dragbike engines and raced for 20 years. Before drag racing I restored British cars. I found a TR3A in a garage and decided to try my hand at the british cars again.
The only reason I mention this is because I am familiar with engines but not the advance feature. All of the drag engines had fixed timing unless using an after market ignition ECU. I always used static timing on the bike engines. I wasn't sure if the timing mentioned in the book for stock was static or running timing.
Anyway, you think I should set the timing at 4 deg static?
I can buy an advance tuning kit for the distributor after I get the carbs adjusted.
Also the carb tuning can be set by vacuum correct? I used to set the synch by sticking a known thickness pin under the butterfly and then setting both to have the same drag on the pin. Will the pin method work on the SU'sThanks for all the advice. This is a great sight...

 

[Brosky]

<div class="ubbcode-block"><div class="ubbcode-header">Quote:]I used to set the sync by sticking a known thickness pin under the butterfly and then setting both to have the same drag on the pin. [/QUOTE]

I hadn't heard that one in a while! It should work, but I've never done it myself. I'm lazy and use a Synchrometer, but I remember some of the senior guys who worked on the Alfa's and Fiat's talking about that method many years ago.

Good luck and welcome to the forum!!

 

[RonMacPherson]

Larry, do you have access to a variable rate timing light?

One that shows you what the timing advances to at various speeds?

That might help with setting the timing advance. You want total timing(on a stock compression ratio engine) to be no more than 32-34 degrees at 3200 rpm.

If you can alter the bottom end advance curve to start kicking in from just above 800 to all in by 1800 you're not going to have any advance problems.

I use the unisyn and highly recommend it.

 

[martx-5]

Anyway, you think I should set the timing at 4 deg static?

Yes...but...

The procedure is simple if you had the original Lucas distributor. The timing mark on the TR3 is at TDC. Timing is set statically at TDC, then the knurled nut on the distributor is turned to advance the timing. I suppose you could figure out where 4 deg BTDC is on the pulley, but an educated guess should get you in the ballpark...maybe 3/8"? As long as the timing is pretty close, it won't be the cause of backfiring through the carb, which is your main concern at the moment.

I would carefully scrutinize the ignition system. Cap, rotor, wires, plugs. Especially the cap, as it might be crossfiring, and recheck the firing order.