You didn't say if you checked that Vic, it would only take a couple of minutes to take the fuel pipe off the carb and time how long it takes to run a certain amount of fuel into a container. I can't remember the timing but it's something like 45 seconds for 1 pint, Stickies book or someone else on here will confirm.bristolmod wrote:Had similar on my TV175- tracked it down to a semi clogged fuel tap. Worth changing/ checking.
Wont rev out and misfiring - carb issue?
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coaster
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Adam_Winstone
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^… true, or the issue may still turn out to be related to ignition or t’other factor.
If this does prove to be induction/jetting related – symptoms of a motor that is being starved of incoming mixture are that the motor will always seem choked and fail to rev out cleanly or make any real power. I came across this when a mate built up a Rapido Classic 200, AF clubman and 26mm carb that he wanted to use with the standard air box/filter with a slope back seat. He took the bike to have it dyno’ed and the inexperienced dyno operator didn’t pick up on the fact that the seat was not letting it breathe correctly (hinge all but blocked off the air scoop passage). As this gave the exhaust readings of it being rich so the jetting was made weaker and weaker until the figures were something like right (!). Even though this motor had over 1000 miles of running in completed, he seized it on the way home from the dyno centre!
My mate came to me and I immediately saw that the motor was not getting enough mixture in, regardless of fuel to air ratio (not enough of either!), and fitted a remote filter for him to try. I immediately went back to the pre-dyno richer jetting and the difference in the bike’s performance was like night and day! With the motor now getting as much of the mixture as it wanted, it revved and performed as expected. It was then dyno’ed by someone else and gave years of excellent service. This really was a great example that it is not merely a case of getting the air/fuel ratio right BUT that you must have enough of both!
If you are using a genuine TV cylinder (or one with any porting) you should note that the TV cylinder already has sufficient porting spec’ for the air intake to be a limiting factor as standard! This is certainly the case of the SIII TV175 cylinder and is why the 60s Lambretta Tuning and Performance Manual does not give any porting modifications but does list extensive modifications to the air box and inlet tract to allow more air into the motor, even with the SIII spec’ air intake. Unlike the 150 and 200 models, for which it lists raising the exhaust port and lowering the inlet, the TV175 porting needs no modification… the intake is the problem area.
There are considerable differences in TV barrels, with aftermarket and over-bored 150s just adding to the differences, and differences in Innocenti’s range of big/little carbs and intakes that they fitted to TVI-IIIs so it is hard to know exactly how your bike may be limited. It is quite right that this may prove to be another issue that is causing the problem. Unfortunately, it is very hard for us to accurately identify the cause without testing the bike in person.
Good luck.
Adam
If this does prove to be induction/jetting related – symptoms of a motor that is being starved of incoming mixture are that the motor will always seem choked and fail to rev out cleanly or make any real power. I came across this when a mate built up a Rapido Classic 200, AF clubman and 26mm carb that he wanted to use with the standard air box/filter with a slope back seat. He took the bike to have it dyno’ed and the inexperienced dyno operator didn’t pick up on the fact that the seat was not letting it breathe correctly (hinge all but blocked off the air scoop passage). As this gave the exhaust readings of it being rich so the jetting was made weaker and weaker until the figures were something like right (!). Even though this motor had over 1000 miles of running in completed, he seized it on the way home from the dyno centre!
My mate came to me and I immediately saw that the motor was not getting enough mixture in, regardless of fuel to air ratio (not enough of either!), and fitted a remote filter for him to try. I immediately went back to the pre-dyno richer jetting and the difference in the bike’s performance was like night and day! With the motor now getting as much of the mixture as it wanted, it revved and performed as expected. It was then dyno’ed by someone else and gave years of excellent service. This really was a great example that it is not merely a case of getting the air/fuel ratio right BUT that you must have enough of both!
If you are using a genuine TV cylinder (or one with any porting) you should note that the TV cylinder already has sufficient porting spec’ for the air intake to be a limiting factor as standard! This is certainly the case of the SIII TV175 cylinder and is why the 60s Lambretta Tuning and Performance Manual does not give any porting modifications but does list extensive modifications to the air box and inlet tract to allow more air into the motor, even with the SIII spec’ air intake. Unlike the 150 and 200 models, for which it lists raising the exhaust port and lowering the inlet, the TV175 porting needs no modification… the intake is the problem area.
There are considerable differences in TV barrels, with aftermarket and over-bored 150s just adding to the differences, and differences in Innocenti’s range of big/little carbs and intakes that they fitted to TVI-IIIs so it is hard to know exactly how your bike may be limited. It is quite right that this may prove to be another issue that is causing the problem. Unfortunately, it is very hard for us to accurately identify the cause without testing the bike in person.
Good luck.
Adam
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victor
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I think the jetting is now close to spot on, but strongly suspect lack of air (or not enough of fuel supply - fast flow fuel tap not flowing fast enough/breather hole in cap clogged...).
I know that running without airfilter leans out the mix but could it be done for a couple of short bursts just to verify (or exclude) the possibility?
I know that running without airfilter leans out the mix but could it be done for a couple of short bursts just to verify (or exclude) the possibility?
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Adam_Winstone
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You could try it but any such dramatic change really does put the motor at risk and must be done with caution! The other issue is that the filter may not be the restriction, rather, it may be a bottle-neck caused by another restriction in the inlet tract. Removing the filter may allow you to test the theory.
Are you doing lots of 'choke tests' whilst trying out your adjustments? If not, do so... reading up on them if you need to.
Do be aware that your jetting may be 'good' for your restriction but that it might be miles out if you make dramatic changes to the inlet tract.
Adam
Are you doing lots of 'choke tests' whilst trying out your adjustments? If not, do so... reading up on them if you need to.
Do be aware that your jetting may be 'good' for your restriction but that it might be miles out if you make dramatic changes to the inlet tract.
Adam
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victor
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With 116 the engine now pulls cleanly throughout the rev range, with only the "coughing" on the absolute top revs remaining.
The last 1000 miles I've been running on 112 main jet, standard pilot and starter and about 1/2 to 3/4 richer airmix than default setting.
118 felt stronger but also a bit rich on the mid range, whereas 120 was too rich all together.
Plug colour is lightly brown, and I have no problems starting or with the tickover.
-> As I mentioned I'll be changing cylinder and with that the carb as well shortly, and will follow up with a dyno test to make sure it goes well, but I thought the dyno could only show you the difference in power outtake with different jettings but not tell you whether you are actually running to lean or not - is this correct?
-> Might be a stupid question but what is the "choke test"? Might have done it but not sure...
-> Finally, what would you suggest be the best way to prove whether the problem comes down to lack of air or not...?
Many thanks V
The last 1000 miles I've been running on 112 main jet, standard pilot and starter and about 1/2 to 3/4 richer airmix than default setting.
118 felt stronger but also a bit rich on the mid range, whereas 120 was too rich all together.
Plug colour is lightly brown, and I have no problems starting or with the tickover.
-> As I mentioned I'll be changing cylinder and with that the carb as well shortly, and will follow up with a dyno test to make sure it goes well, but I thought the dyno could only show you the difference in power outtake with different jettings but not tell you whether you are actually running to lean or not - is this correct?
-> Might be a stupid question but what is the "choke test"? Might have done it but not sure...
-> Finally, what would you suggest be the best way to prove whether the problem comes down to lack of air or not...?
Many thanks V
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coaster
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Vic, I'm sure you said earlier that your air intake is un-modifie (standard) TV 175 s3. As Adama says if you intend running the current setup OR the intended MB setup then you MUST carry out the modifications to the scoop and airbox.
The choke test is simple, just try turning it on at different points in the rev range. If the performance improves then it points to a weak mixture and if it bogs down it wouyld indicate a rich mixture. It isn't very scientific and tbh I haven't had much success when I've tried it.
Regards dyno's, a skilled operator will be able to set the jetting by measuring the air/fuel ratio (as mentioned by Adam), ideally he should back this upo with a road test. I have no idea of the closest one to you, I think there is one in Southend but the one run by Diablo on here up in Nuneaton is highly recommended. Could be worth hiring a van at some point.
The choke test is simple, just try turning it on at different points in the rev range. If the performance improves then it points to a weak mixture and if it bogs down it wouyld indicate a rich mixture. It isn't very scientific and tbh I haven't had much success when I've tried it.
Regards dyno's, a skilled operator will be able to set the jetting by measuring the air/fuel ratio (as mentioned by Adam), ideally he should back this upo with a road test. I have no idea of the closest one to you, I think there is one in Southend but the one run by Diablo on here up in Nuneaton is highly recommended. Could be worth hiring a van at some point.
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victor
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Hi Coaster, many thanks for your input 
I believe that we are agreeing that it might choke on too little air - there was a reason why innocenti changed design of the airbox even for standard setup and considering that mine now is higher revving that probably results in not enough air flowing through... Question is if I can test this theory in any simple way...?
There should be a place in Essex which has helped Scootering out when they have tested exhausts for performance - will bring it there once finished with the MB setup, if no one else recommends a closer and better garage...
My intention with the MB setup is to not use the airbox but to go with foam filter under the panel and use the space from the airbox for a bigger tankcoaster wrote:Vic, I'm sure you said earlier that your air intake is un-modifie (standard) TV 175 s3. As Adama says if you intend running the current setup OR the intended MB setup then you MUST carry out the modifications to the scoop and airbox.
In that case I've tried it and it worsens considerably. I'm 100% sure the 116-118 is fine, haven't tested 120 but think that's where it starts to run too rich considering how severely rich it ran with 122.The choke test is simple, just try turning it on at different points in the rev range. If the performance improves then it points to a weak mixture and if it bogs down it wouyld indicate a rich mixture. It isn't very scientific and tbh I haven't had much success when I've tried it.
I believe that we are agreeing that it might choke on too little air - there was a reason why innocenti changed design of the airbox even for standard setup and considering that mine now is higher revving that probably results in not enough air flowing through... Question is if I can test this theory in any simple way...?
How do they measure the air/fuel ratio, is something clamped on in the fuel-air line to measure this, or is it measured in the exhaust fumes...? Sorry for these ridiculously stupid questions but not having been present at a Dyno run yet I really have no clue...Regards dyno's, a skilled operator will be able to set the jetting by measuring the air/fuel ratio (as mentioned by Adam), ideally he should back this upo with a road test. I have no idea of the closest one to you, I think there is one in Southend but the one run by Diablo on here up in Nuneaton is highly recommended. Could be worth hiring a van at some point.
There should be a place in Essex which has helped Scootering out when they have tested exhausts for performance - will bring it there once finished with the MB setup, if no one else recommends a closer and better garage...
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Adam_Winstone
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measured in the exhaust fumes
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victor
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I can't really see any reason for me to use the original air filter setup considering I have the limiting box and the intake under the seat hasn't been taken out...
Are there any inexpensive foam filter setups that can be used with the the SH1/20 delly?
Would that be recommendable...?
Will see if I can get the fuel tap test done tonight
Are there any inexpensive foam filter setups that can be used with the the SH1/20 delly?
Would that be recommendable...?
Will see if I can get the fuel tap test done tonight
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victor
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So, installed a PWK 28mm BGM carb with foam filter, and the engine is much more responsive and has slightly more power.
However it still cuts off on really high revs so the problem lies elsewhere.
Going to check points and timing tomorrow and perhaps set it closer to 18 degrees than 19 I should currently have.
Could it be as simple as points bouncing? If so, are there any really strong points that can be sourced? What is the difference between the early type points and the later GP ones?
And the work swapping carb has begun by wheredowegonowbutnowhere, on Flickr
However it still cuts off on really high revs so the problem lies elsewhere.
Going to check points and timing tomorrow and perhaps set it closer to 18 degrees than 19 I should currently have.
Could it be as simple as points bouncing? If so, are there any really strong points that can be sourced? What is the difference between the early type points and the later GP ones?
And the work swapping carb has begun by wheredowegonowbutnowhere, on Flickr