Underfueling after carb rebuild

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Underfueling after carb rebuild

Postby pearcedj » Sat Mar 13, 2021 2:31 am

Hi All,

Firstly, thanks to the administrators and contributors who have made this forum such a great resource. I am in Melbourne and have a '97 green TRX that I have been slowly rebuilding over the last 12 months. It was in fairly rough condition, including bent forks, broken fender, mice in the tail section, a glued on fleece seat cover and homemade brake rotors! I have no mechanical experience, so have been reading the forum extensively and so far it has the usual recommended mods/upgrades: changed to an on/of fuel tap, new running gear bearings and seals, rebuilt suspension by MPE, Metalgear rotors and rebuilt blue spots/14mm MC, new OEM plug caps and inlet manifold rubbers, new spark plugs, Kedo right air cut cover, new air filter, and Ignitech ignition. Most recently I have attempted rebuilding the carbs with Keyster kits and LiteTek seals. I have also blocked off the left inlet manifold vacuum hose closed to the manifold.

Prior to the rebuild (and with the old manifold rubbers) the bike was snatchy at low RPM and would pop on overrun. On opening the carburetors, the emulsion tubes and needles were worn and many seals perished. I put the Keyster needles and emulsion tubes in, and also modified the teflon washer on the needle by dremmeling an edge down so that the needle does not rub on the emulsion tube as much. Visually, the Keyster needle appeared to have the same taper as the Mikuni ones and also had the same number of clip positions - 3. All jets were switched out to the standard factory sizes provided in the Keyster kit, i.e. 142.5 mains. I set the needle clip to the middle position, pilot screws to 2.5 turns out. So now I have the same needle clip position as before, but with the needle sitting deeper due to the modified washer.

It started and idles well, however when revving up to 4,500-5,000 rpm it seems to run out of fuel and the rpm drops, accompanied by the odd pop and bang. When riding the bike, it completely dies if I open the throttle past 3,000 rpm, and even lower if there is more load on it. When cracking the choke one click: it runs like a dogs breakfast (surging) with the choke at low rpm, happy around 3,000, and stalls at idle. I have opened the carbs again and flushed all the jets with minimal improvement. I am 100% certain fuel is reaching the carbs (my first fuel line would kink when it got warm), and I do not see how it could be leaking air in with all the new seals and rubbers in place, so is it most likely to be an issue with my needle/clip position? I have not had a chance to try the lowest clip position yet, but my gut feeling is that changing one needle clip position would not resolve such severe symptoms.

Other thoughts I have had:
-could it be an issue with the TPS? I did try checking position the other day, but stupidly (I think!) had the Ignitech hooked up and so the tacho never moved. Resistance seems within spec on the multimeter though.
-could there be an air leak somewhere else causing these symptoms, for example the other vacuum hose from the fuel pump to the right manifold?
-Have I just completely buggered up the carbs or forgotten to connect something?!

Thank in advance for any thoughts or suggestions. I realise that this is a "lazy" post because there are still things I can try, but the bike is kept over an hour away and I have very limited time to work on it - so being able to direct my efforts efficiently would be great.

Cheers,
Duncan
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Re: Underfueling after carb rebuild

Postby dicky » Sat Mar 13, 2021 5:22 am

Dropping the needle further down the tube will lean out the mixture.
Try getting the needle back to the same position it was prior to modifying the washer.
You can use the Ignitech software to measure the TPS voltage, a good way to see if it is operating correctly.
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Re: Underfueling after carb rebuild

Postby Rod.s » Sun Mar 14, 2021 2:42 am

This has all the hallmarks of a fuelling issue, ignition issues are either permanent, cyclic or heat related. Coils for example will work fine when cold but once hot will break down from higher resistance.

The TPS is just there to advance the ignition on quick throttle openings, at this point the engine is flooded with excess fuel and the ignition needs to be advanced to start the burn well before TDC giving better performance. If you are testing the engine the TPS will only make a small difference to normal throttle openings and not give the effect of "when revving up to 4,500-5,000 rpm it seems to run out of fuel and the rpm drops, accompanied by the odd pop and bang".

# Have you removed the "Pilot Merge Cavity bung" and cleaned it?
I've taken this from https://litetek.co/Guide_MikuniBDST_Reb ... 4#MainJets
Image

# when you say that you have dremeled the plastic ring, do you mean that you have cut the edge off or just put a step in it? either way you will have lowered the needle by that amount, so the needle will need to be raised to account for this. If you have just dremeled a step in the plastic ring, for purely testing purposes remove it and flip it over to take the needle back to factory settings.
# Set the idle pilot screw at 2 turns out (factory recommended).
Image

#Where the fuel line enters the carb's there is a "T" piece that has a fine mesh filter fitted, remove it and check it's not blocked. Some remove it altogether and then fit a clear inline filter close to the fuel pump so it can be checked.

# do you have a quality carb balancer? if you have rebuilt the carbs the balance will be changed, once the issue is sorted balance the carbs..

lastly when the engine is hot and idling, spray some WD40 (using the straw) around the inlet manifold and note any changes in the engine RPM. This will tell you if there is an air leak.

Cheers
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Re: Underfueling after carb rebuild

Postby pearcedj » Sun Mar 14, 2021 3:39 am

Hi Dicky and Rod,

Thank you for your swift advice!

# Yes, I did remove and clean the pilot merge cavity bungs. I'll clean them again; I may have been too heavy handed with silicone spray and caused a blockage.

# I did dremel an entire side off the teflon washer. I will try raising the needle back to where it used to live and test it there. As a side note, do any rebuild kits come with new washers? I feel stupid now that I may have ruined my set up...

# No, I didn't know there was a filter in the T-piece - thanks, I'll clean it! I assume it is OK to put a filter between the pump and carbs? I tried fitting one between the tap and pump, but found it a bit cramped and my fuel hose was kinking.

# And no, I'll need to invest in a carb balancer.

Cheers,
Duncan
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Re: Underfueling after carb rebuild

Postby Rod.s » Sun Mar 14, 2021 9:52 am

Ok what your are experiencing is under fuelling, raise the needle back to normal and start the tune from there.

Sorry but the kits don't have the white plastic ring..

The pilot jets is for idle up to 1500-1750 rpm, the needle starts working from there until it is clear of the main jet (around 3/4 throttle) from there to red line it's all main jet etc....

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Re: Underfueling after carb rebuild

Postby dicky » Sun Mar 14, 2021 10:40 pm

To give a bit more wiggle room under the tank, try fitting a long(er) length of fuel line from the tap to the pump, and coil it in a loop.
This way you can leave the fuel line attached to the tank and still be able to raise the front of the tank on the rear pivot bolt.
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Re: Underfueling after carb rebuild

Postby pearcedj » Tue Mar 23, 2021 11:35 pm

Moving the needle further out of the emulsion tube did the trick, thank you very much! I also went back to stock pilot screws 2 turns out.
Some rough measuring along the way, to try and work out how much the washer mod (that I understand quite a few people here do) affects the position:

Clip position.....Washer modified.....O-ring......Needle length mm....Outcome
Middle (2)........No...................... No..........43.5
Middle (2)........Yes......................No..........45.0.....................Too lean
Richest (3).......Yes......................No..........44.0.....................Too lean
Richest (3).......Yes......................Yes.........42.25-42.5..............Runs!
Middle (2)........Yes......................Yes.........43.5.....................Haven’t tried it

At the richest clip position with the modified washer, the bike still seemed too lean - it would falter above 5k rpm (I didn't ride it). I then put some spare O-rings from the carb rebuild (LiteTek number OR-001) between the clip and washers which lifted the needle another 1.5mm or so, and the bike is a least rideable again :D . It falters a little around 2500-3000 rpm when rolling on, but is good up high. I'm not good enough to know if it is still lean or now too rich in the mid range; I will check the plugs and play around with the position some (and balance them when my fancy balancer arrives!). Perhaps using the O-ring isn't a great long term solution as I imagine it may swell and perish over time.

I guess for me, this brings into question whether the washer mod (viewtopic.php?f=37&t=12174&p=119873&hilit=needle+washer#p119873) works with new needles and tubes, or perhaps in particular the Keyster ones. Are there other kits come with more needle clip positions - e.g. 5 instead of 3? The old ones I took out of my carbs had the Mikuni stamp and 3 positions.
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