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The bike has carbs and standard engine with the AI equipment removed. Uni-filter fitted with Norman Hyde Bellmouth on the air box and Toga silencers. Stock ignition. Carbs have Thruxton needles with one shim and the slide is drilled to 3mm air hole. Main jets are 128 and Pilot jets are 40. Mixture screws; left = 3 3/4 out and right = 3 1/2 out.
When trying to start it fires and tries to run but then cuts out with half choke. Tried various combinations of more or less choke and adjusting the mixture screws in and out but without an improvement.
After many, many attempts it does eventually keep running and then runs ok with good response in all revs and good top speed. Also when hot starts straight away with no choke or hesitation.

When first got the bike it had 125 Main jets and 40 Pilot jets but ran like a "stuffed pig", so installed 128 Main jets and 42 Pilot jets and immediately ran better but would only start with no choke and only after many attempts. Thus I changed the pilot jets down to 40 and at least tries to run now with half choke but takes a very long time to keep running.

Any suggestions please?
 

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The bike has carbs and standard engine with the AI equipment removed. Uni-filter fitted with Norman Hyde Bellmouth on the air box and Toga silencers. Stock ignition. Carbs have Thruxton needles with one shim and the slide is drilled to 3mm air hole. Main jets are 128 and Pilot jets are 40. Mixture screws; left = 3 3/4 out and right = 3 1/2 out.
When trying to start it fires and tries to run but then cuts out with half choke. Tried various combinations of more or less choke and adjusting the mixture screws in and out but without an improvement.
After many, many attempts it does eventually keep running and then runs ok with good response in all revs and good top speed. Also when hot starts straight away with no choke or hesitation.

When first got the bike it had 125 Main jets and 40 Pilot jets but ran like a "stuffed pig", so installed 128 Main jets and 42 Pilot jets and immediately ran better but would only start with no choke and only after many attempts. Thus I changed the pilot jets down to 40 and at least tries to run now with half choke but takes a very long time to keep running.

Any suggestions please?
Hi

I鈥檝e just about finished messing with my 790s jetting and have 132 main with 42 pilot mixture screws 2 turns out with standard needles (1 shim) performance filter, bell mouth and mapped ignition.

I take it it won鈥檛 start on choke? I also take it it hasn鈥檛 got worse?
It sounds as though your mixture is rich.
I would pop your mixture screws back to standard (2 1/2) turns out and go from there.
Whatever you do, do one thing at a time (so you can see what changes) and make sure you have a good blue spark and keep the battery charged as you go.

Hope this helps

Chris




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Hi


It sounds as though your mixture is rich.
I would pop your mixture screws back to standard (2 1/2) turns out and go from there.
Whatever you do, do one thing at a time (so you can see what changes) and make sure you have a good blue spark and keep the battery charged as you go.

CHECK THE SPARK AT THE PLUGS It may be getting weak spark which won't fire a cold mixture/engine. So many carb issues turn out to be weak ignition and the owner ends up running down a rabbit hole. (I have in the past, that's for sure). Does it have the stock coil? It may be getting long in the tooth and not up to par. When I changed my coil (after it totally failed) many of my cranking problems disappeared. That and a new battery.




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That鈥檚 why I mentioned the blue spark mate, definitely worth emphasising though..


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Discussion Starter #5
Hi

I鈥檝e just about finished messing with my 790s jetting and have 132 main with 42 pilot mixture screws 2 turns out with standard needles (1 shim) performance filter, bell mouth and mapped ignition.

I take it it won鈥檛 start on choke? I also take it it hasn鈥檛 got worse?
It sounds as though your mixture is rich.
I would pop your mixture screws back to standard (2 1/2) turns out and go from there.
Whatever you do, do one thing at a time (so you can see what changes) and make sure you have a good blue spark and keep the battery charged as you go.

Hope this helps

Chris




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Thanks for your reply Chris.
It will start on 1/4 to 1/2 choke but only after many attempts turning it over. It has actually got better since I reduced the Pilot jets from 42 to 40. With 42's it would only start without choke, but again only after many attempts. That's why I went down to 40's.
I have returned the mixture screws to 2 1/4 and 2 but get worse results; it fires but won't attempt to run. That's why i increased mixture screws up in stages to 3 3/4 and 3 1/2 where it tries to run but cuts out.
I'm keeping a log of everything I do and the results for reference.
The last time I checked, the sparks were good (with new plugs) and no soot or soaking in petrol. The battery is also newish - 2017. It's always on an Optimate as well.

What do you think about going down to 125 Main jets? (If you think it's too rich).
 

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Discussion Starter #6
Thanks for your reply.
The coil is stock and would be from 2003 - so 16 years old. The bike has 35,590 miles on the clock.
 

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Thanks for your reply Chris.
It will start on 1/4 to 1/2 choke but only after many attempts turning it over. It has actually got better since I reduced the Pilot jets from 42 to 40. With 42's it would only start without choke, but again only after many attempts. That's why I went down to 40's.
I have returned the mixture screws to 2 1/4 and 2 but get worse results; it fires but won't attempt to run. That's why i increased mixture screws up in stages to 3 3/4 and 3 1/2 where it tries to run but cuts out.
I'm keeping a log of everything I do and the results for reference.
The last time I checked, the sparks were good (with new plugs) and no soot or soaking in petrol. The battery is also newish - 2017. It's always on an Optimate as well.

What do you think about going down to 125 Main jets? (If you think it's too rich).
So once warm the fuelling is good? What is the ambient temp (approx) where you are? Do the carb heaters work?

Have you removed the baffle in the airbox?

So it still sounds like the bike is rich, what type of air filter is fitted?

I鈥檓 interested in the use of thrux needles, when did you fit them and how did it run before fitting?

It takes a bit of messing about to get there.

Chris


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Discussion Starter #8
So once warm the fuelling is good? What is the ambient temp (approx) where you are? Do the carb heaters work?

Have you removed the baffle in the airbox?

So it still sounds like the bike is rich, what type of air filter is fitted?

I鈥檓 interested in the use of thrux needles, when did you fit them and how did it run before fitting?

It takes a bit of messing about to get there.

Chris


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Hi Chris,
When the engine is warm it runs great and no problems at all and it will start at the touch of the button with no choke.

The temperature at the moment is about 10 C. I haven't checked the carb heaters but I had this starting problem back in June when the temperature must have been nearly double that (although I had 42 Pilot jets in at that time).

The baffle is still in the airbox.

The air filter is the "Uni-filter" make which is the high flow one.

The Thruxton needles were already fitted when i got the bike, but I knew it ran well before hand.

I know it takes time to tune an engine, but I feel at the moment I'm going around in circles and came on here in the hope someone could break that circle for me and point me in the right direction.

Cheers.
 

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Hi Chris,
When the engine is warm it runs great and no problems at all and it will start at the touch of the button with no choke.

The temperature at the moment is about 10 C. I haven't checked the carb heaters but I had this starting problem back in June when the temperature must have been nearly double that (although I had 42 Pilot jets in at that time).

The baffle is still in the airbox.

The air filter is the "Uni-filter" make which is the high flow one.

The Thruxton needles were already fitted when i got the bike, but I knew it ran well before hand.

I know it takes time to tune an engine, but I feel at the moment I'm going around in circles and came on here in the hope someone could break that circle for me and point me in the right direction.

Cheers.
Yeah I agree it is frustrating..

You should check your fuel levels in the carb bowls. Use a clear pipe from the drain and lift it parallel to the bowl, then open the drain.

so your mixture sounds rich to me.

I would reduce the main and reset the mixture screws (you already have a standard idle jet).

I hope you get there mate

Chris


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If it's not being too nosy, where are you located?
 

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Seems like you may have a pilot circuit problem. Try running some Seafome or carb/injector cleaner of your choice through a tank of gas. Check for air leaks between the carbs and the head, boots,''O'' rings etc. If no success remove the float bowls, pilot jets and D shaped idle air screw. Using the straw that comes with a can of spray carb cleaner, spray through all fuel and air passages.
 

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Discussion Starter #12
Yeah I agree it is frustrating..

You should check your fuel levels in the carb bowls. Use a clear pipe from the drain and lift it parallel to the bowl, then open the drain.

so your mixture sounds rich to me.

I would reduce the main and reset the mixture screws (you already have a standard idle jet).

I hope you get there mate

Chris


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Thanks for ideas.

I will check the fuel levels and reduce the main jets to 125 and reset the mixture screws and do the other checks suggested..

But I will wait until the festivities and New Years celebrations are over first as I will not be in any fit state to do anything sensible otherwise!

Cheers.
 

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Discussion Starter #14
Seems like you may have a pilot circuit problem. Try running some Seafome or carb/injector cleaner of your choice through a tank of gas. Check for air leaks between the carbs and the head, boots,''O'' rings etc. If no success remove the float bowls, pilot jets and D shaped idle air screw. Using the straw that comes with a can of spray carb cleaner, spray through all fuel and air passages.
Thanks for your reply.

I'll check for any air leaks and spray some carb cleaner through the carb passages.

Cheers.
 

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Hi!
I'm in South West London, of the UK.

Cheers.
Ok I thought if you were anywhere near the lowcountry of SC in the states I'd be happy to drive over and give you a hand troubleshooting things by swapping parts to eliminate things. Good luck.
 

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Discussion Starter #16
Ok I thought if you were anywhere near the lowcountry of SC in the states I'd be happy to drive over and give you a hand troubleshooting things by swapping parts to eliminate things. Good luck.
Ok thanks for the offer, but a bit too far to travel I think!
 

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Discussion Starter #17
Thanks for ideas.

I will check the fuel levels and reduce the main jets to 125 and reset the mixture screws and do the other checks suggested..

But I will wait until the festivities and New Years celebrations are over first as I will not be in any fit state to do anything sensible otherwise!

Cheers.
Here's the results of the first tests that I've done;-

1.Carb heaters; Temperature measured with a temperature "pen" was 9 - 10C and heaters were not working (no voltage on wire connections).

2.Fuel level; 2mm above float mating surface on both carbs (correct to manual).

3.Plug colour; Both had blue sparks, but I thought looked a bit small? Not sure. (Maybe clutching at straws here. Thinking of getting a new coil and trying for a comparison).

4.Carb air leaks; Checked all around rubber boots and seals, re-tightened clamps and no leaks found.

That's all so far.
Next step, as an experiment I thought I would remove air filter, replace bellmouth and side panel and attempt to start engine to see what happens.

More to follow.
 

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Check and make sure the carbs are balanced. Out of balance carbs can cause a cold engine to be hard, slow, to start and be very cold natured, with little to no noticeable effect on a warm engine. A main jet one or two sizes off or a needle change probably wouldn't affect starting as much as idle circuit and/or carb balance.
 

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Discussion Starter #19
Check and make sure the carbs are balanced. Out of balance carbs can cause a cold engine to be hard, slow, to start and be very cold natured, with little to no noticeable effect on a warm engine. A main jet one or two sizes off or a needle change probably wouldn't affect starting as much as idle circuit and/or carb balance.
Thanks for info.
I had balanced the carbs some time ago when I initially got the bike running and I have no reason to suspect they have gone out of balance now, but It's another thing to check when I do get it started again.
 

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Discussion Starter #20
Here's the results of the first tests that I've done;-

1.Carb heaters; Temperature measured with a temperature "pen" was 9 - 10C and heaters were not working (no voltage on wire connections).

2.Fuel level; 2mm above float mating surface on both carbs (correct to manual).

3.Plug colour; Both had blue sparks, but I thought looked a bit small? Not sure. (Maybe clutching at straws here. Thinking of getting a new coil and trying for a comparison).

4.Carb air leaks; Checked all around rubber boots and seals, re-tightened clamps and no leaks found.

That's all so far.
Next step, as an experiment I thought I would remove air filter, replace bellmouth and side panel and attempt to start engine to see what happens.

More to follow.
5. With the air filter removed and bellmouth and side panel back on I get the same results with various choke positions - the engine fires, tries to run and cuts out.
(Air filter now put back).

Next weekend I will change the main jet to 125, reset the mixture screws and carry on.
 
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