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Common Carb Issues

227915 Views 133 Replies 74 Participants Last post by  latebird
Just trying to get a list of common carb issues for a 2002 TB.

From what I was able to get from the parts guy from the dealer was the following.

- The needle and seat was a common part to wear out which hinders the carbs ability to prevent fuel from back-flowing out and getting into the air cleaner.

- The pilot jet. Mentioned that it can be cleaned but at $6 it is not a bad idea to replace it.

I am currently trying to resolve a car issue I have right now and would love the feedback.

Please let me know of any other common carb issues. Once I get a bunch of isses I can post the issues and cause.
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i have a quick question you guys really seem to know what your talking about so i have an 95 trail boss 2 stroke not getting oil can i just plug the oil lines and mix the gas myself and pour it in i would rather do it that way any way
I recommend anyone with a two stroke to plug the oil tank and always mix the gas and oil then put in the fuel tank. I had a jet ski that had a oil tank and mixed automatically. After the oil pump went out and the engine didnt recieve any oil it caused the engine to lock up. This cost me about 2800$. I agreed with the mechanic and we plugged the oil tank line and I always pre-mixed because then you never have to worry about it you are getting oil and the correct amount mixed before injecting into the engine. So, again I completely agree with plugging line and always pre-mixing. Happy Riding.
The only time I've ever seen a two stroke oil pump fail was when it was left sitting with an oil/water mix for about 2 yrs.
You need to re-jet if you use pre-mix, the oil takes the plce of fuel and does cause slightly lean mixture, plus, the extra oil at lower rpm tends to collect in crankcase and exhaust (plus oil only creates heat, not power)
It isn't always possible to use pre-mix on all two stroke engines without major work, some need cranks disassembled to remove oil slinger plates (they direct oil to big end bearing from one main bearing)
You did check and re-set float level? It ALWAYS needs checking when you fit new parts, OEM or aftermarket
The air jets will be on air box side of carb or under diaphragm on most CV carbs,
Saying' intake side' doesn't actually tell anyone anything as carb is on intake side of engine and direction of airflow is from air box to engine (mostly).
Due to reverse airflow at idle (valve overlap at TDC) some 'spitting' is inevitable, but, it is usually accounted for when entire air system is in place (from filter inlet right through to valve head) If fuel level in float bowl is too high (float height too low) the pulses will pressurise float bowl and 'blow' fuel out of lowest points. It can get bad if various vent holes and hoses are not clear
He stated fuel is coming out of air jet, if float was stuck it will come out of breather, overflow and various jet discharge ports. Highly unlikely to be dirt in shut off valve. On Mikuni carbs (and a few others) the air bleeds in intake (air box side) are lower than discharge ports. Early Mikuni (VM, etc) have drilling directly to emulsion tube (or primary choke) which is lower than the actual fuel discharge ports. It's most likely OP assumed a genuine repair kit didn't need float level checked and just installed new parts (or checked float level incorrectly?) I taught this stuff at MMI and every dealer workshop I ran before that.
There is nothing wrong with trying to help, but read at least some of the previous posts first (I didn't read all118 previous as this problem was 'recent' and not original 2010
My best suggestion is to post a picture of the 'offending' part.
It would give us a much better idea of what the problem is.
Glad to hear you have had a successful business since 1978, I was working as a precision machinist then (trained so I could make BSA parts fit my Royal Enfields, BSA parts were much easier to get hold of.)
I've got various carbs here but grandson has run off with camera (don't like using phone)
Pretty much everything since mid 70's EPA mandates will be a little different
Mikuni's on a 1968 Suzuki T305 (worked on one Saturday so it's first thing that came to mind) are almost the same as the VM's sold today but 'New' CV carbs are very different (Harley kinda messed things up by having Mikuni and Kei-Hin brand carbs as HD and removing all external and most internal brand markings)
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Oh, here's an old Kei-Hin (mid 70's) The air box connects to left side and carb connects to engine on right side.
The model and type will be on carb somewhere although Mikuni tend to have very ligfht etch or 'print' as it's cheaper than stamping. I've worked on just about any carb you can think of (including Jikov, Bing and HD 'chainsaw' carbs)
It may not be easy to 'fix' a carb long distance but it wouldn't be the first or last time I've done it
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I'll find some pics.
Your probably as old as me so know there are few 'new' techs/mechanics that understand carbs and want to plug 'everything' into a computer for 'diagnostics'
I've pretty much exclusively worked on bikes since 1968, I know plenty of 'factory trained' mechanics who can only do things 'by the book' (and are mainly very good at it) but, often you have to think 'outside the box'
One thing I have found in last 5~10 years, float levels need to be at least 1mm higher than stock to compensate for E10 being 'thinner' (less dense) than non alcohol fuels. You may remember test kits being sold to check for alcohol fuels in the 70's?
Personally I think it's evil stuff being foisted on us by farming lobby. It isn't 'cheaper or more cost effective plus results in higher fuel consumption
We still need to know carb make and model. It's 'probably' a CV carb but guessing isn't smartest thing to do?
Anyway, more pics cos' people like them (these are all old stuff but I can find them)
The 'air jet map' wasn't done by me but shows location of air jets and corresponding fuel jets.It's a representation of vacuum chamber on top of carb looking straight down (by Trek97 on DTT)
Honda has used 3 jet carbs since the mid/late 1960's and majority of people STILL don't understand them. (CB250/350 twins, etc) 1978 on they got more 'complicated' and way more difficult to tune or modify from stock settings due to EPA fines (although not as bad as Yamaha's $26 million fine a few years ago)

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OH, the 'map' is actually incorrect, the piece marked 'Intake' should read 'to air box'
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