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4rt repsol. Dropping engine speed


gasgasshetland
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Hi there just wondered if anyone has experienced a problem like I’m having.

my 15 repsol starts fine and runs away fine, but after a bit it starts dropping the engine speed on shutting off the throttle and it goes below idle speed and stalls. It will immediately start again and idle away fine, but drive the bike again and it will drop the idle speed and stall. 

So it’s happy idling from start up, but drops off from shuttling the throttle off. Seems to be once it’s warm. While running it takes full throttle and seems fine, but once shutting off it will stall. 

I have cleaned the throttle body main venturi butterfly  passage and the air passage that is in use when idling.  There had been an alteration done to the bike prior to me buying it and the head breather oil catch tank had been moved to under the seat and wasn’t doing it’s job and oil was getting into the airbox then going through the throttle body.  I have put a small breather filter up under the headstock. 

Its my first montesa 4rt so not really that well read on the injection setup. 

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3 hours ago, gasgasshetland said:

Hi there just wondered if anyone has experienced a problem like I’m having.

my 15 repsol starts fine and runs away fine, but after a bit it starts dropping the engine speed on shutting off the throttle and it goes below idle speed and stalls. It will immediately start again and idle away fine, but drive the bike again and it will drop the idle speed and stall. 

So it’s happy idling from start up, but drops off from shuttling the throttle off. Seems to be once it’s warm. While running it takes full throttle and seems fine, but once shutting off it will stall. 

I have cleaned the throttle body main venturi butterfly  passage and the air passage that is in use when idling.  There had been an alteration done to the bike prior to me buying it and the head breather oil catch tank had been moved to under the seat and wasn’t doing it’s job and oil was getting into the airbox then going through the throttle body.  I have put a small breather filter up under the headstock. 

Its my first montesa 4rt so not really that well read on the injection setup. 

What idle speed have you got it set at?

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I remember reading a post where there was a little play in the PTB and when shutting off the throttle it would go past 0 and stall out, a small drop of super glue under the stop was used to diagnose the issue.  

I don’t remember 100% what the final fix was , but there was some talk about a washer as a shim and a few other options talked about.

not sure if that will help but good luck figuring the true root cause out.

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14 hours ago, gasgasshetland said:

... So it’s happy idling from start up, but drops off from shuttling the throttle off. Seems to be once it’s warm. While running it takes full throttle and seems fine, but once shutting off it will stall. ...

Given the things you've already checked, and that you already corrected the previous owners breather modification, you may want to:

1) Check the throttle cable to make sure it has enough free play per the manual.

2) Adjust the idle up (CCW) a quarter turn. Start, let warm up until fan comes on, blip throttle a few times. The idle should be adjusted after this much warmup. If, when it drops, it then stalls, try again with another quarter turn. See if you can get it to the point where it idles and doesn't stall after the warmup & blipping. Then adjust to proper idle.

 

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13 hours ago, jonnyc21 said:

I remember reading a post where there was a little play in the PTB and when shutting off the throttle it would go past 0 and stall out, a small drop of super glue under the stop was used to diagnose the issue.  

I don’t remember 100% what the final fix was , but there was some talk about a washer as a shim and a few other options talked about.

not sure if that will help but good luck figuring the true root cause out.

Had that on full throttle but not at zero throttle  - I think there maybe a screw for zero position ??

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2 hours ago, jimmyl said:

Had that on full throttle but not at zero throttle  - I think there maybe a screw for zero position ??

There is a stop setscrew, but it's set and painted from the factory. Don't know how to set it. Do you need to be measuring TPS resistance (How? Undocumented pin on the 32P connector? Access ECU board?) and setting based on some value? Or is it just a stop that's set so the butterfly valve doesn't rest hard against the TB bore? Or is the butterfly valve set for soft closure, and then the ECU is calibrated so the TPS is zeroed? I'd be reluctant to start screwing with that screw without more knowledge.

With a used bike, also brings up the question of whether or not the previous owner tried to adjust idle with that screw rather than the air bypass screw. You may want to look and see if the factory paint was broken.

Edited by sportsawyer
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You really need to confirm these settings before goining any further:

Valve clearances set to specification.

Throttle body, air filter hose clamp not ovetightened causing distortion to TB housing.

Honda put the various breathers into the design for a reason put these all back to standard, ditch any extra contrivances as unnecessary.

Ensure all breathers and entry and exits are actually clear.

Engine oil as specified and quantity correct.

Fuel is fresh and fuel filters good.

Fuel pressure to specification.

Engine idle speed to Honda specification.

No air leaks in inlet system or exhaust.

No restrictions in air filter, system.

Throttle cable adjusted as per Honda specs.

Coolant temperature within Honda parameters. 

Cooling fan operates to Honda specs.

Compression test to specifications, not guessed to be ok.

Observations: too tight on the inlet and/or exhaust valve operating clearances will cause unsustainable idle speed.

Coolant temperature above specification will cause erratic idle behaviour.

Excess crankcase pressure will cause engine stalling and erratic idle behaviour.

Poor compression will cause erratic behaviour at idle. Also poor performance in the mid range.

In general it will probably come down to being something trivial being missed so be extra vigilant when making any investigations.

Hope this is helpful and you get sorted.?

 

Edited by section swept
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