49cc 4 stroke bogs down, possible vacuum problem.

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prodiverr

New Member
Jul 8, 2015
6
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Nanaimo
Hi. Start off by saying I know very little about the bike I just bought. It runs great and last time I took it out it was running up to 50 or more kms. Then I put some high end 91 grade gas in it, and started to experience problems. Fires up first pull, but only runs for a bit. I put a different hose on to bypass the pinwheel, that is not the problem. Gas flow to carb is clear. But I can see that the gas is not being sucked into the carb, it stays in the hose. I also noticed that alittle gas was forced out the top, also leading me to believe its a vacuum issue. Before I start taking the carb off, I decided to ask the more experience user here.

So I wait for advice. Please any advice would be appreciated.
 
Trey

Trey

$50 Cruiser
Jan 17, 2013
1,432
5
0
Where cattle outnumber people 3 to 1.
Welcome to the forum.

I think your best bet is to remove, clean and inspect your carb. There's not much to it. Give it a shot!

Good luck, and keep us informed.

Trey

Also, have you signed on to the member map yet?
 
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Slogger

Member
Sep 8, 2014
544
4
18
nohio
In this town, at the old gas station I use, I bought some premium gas for my 2 stroke.
Once. It seems the regular gas sells quickly (is fresh) while the premium sits in the station's tank for months on end (and is funky). My bike would hardly run on it.
Clean and check out the carb, but if the problem persists, try a tank of regular.
 
Davezilla

Davezilla

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Mar 15, 2014
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San Antonio Texas
Another thing to consider with these small engines is that they shouldn't need higher octane fuel unless something was done to give it higher compression. Most engines aren't that picky but some of these small engines can be pretty picky as far as fuel and octane goes. Octane is the fuel's resistance to burning so switching to a higher octane could give it problems where it just don't feel quite as strong as it should, or it could even hesitate or miss depending on how strong or weak the ignition is.
The basic rule of thumb with the fuel as far as octane goes is to use the lowest octane you can without pinging or detonation. I know high octane are advertised all over the place as premium or that this fuel will give the engine just a little more power, but that's just the oil companies way to sell more of it. Think of octane more as a necessary evil when you got a high compression engine where you need to tone it down in order to prevent detonation, kinda like retarding timing or reducing compression or adding octane.... so basically it should be avoided unless needed.
My engines tend to start easier, run smoother, and typically have just that little bit more pep to them when just running regular 87 octane pump gas. I do have to run 93 octane in one engine since it has a 12.7:1 compression ratio but the engines that are a lot closer to stock definitely run better with the lower octane regular gas.
If your engine is stock and if it's picky enough, it could just be the fuel that's giving you the problems.
 
knightscape

knightscape

Member
Jul 29, 2013
340
1
16
Maine
Does it run ok with the gas cap loose? I had a problem with my gas tank that it would develop a vacuum that the vent in the gas cap couldn't release. I'd gas up and get a couple miles down the road before it would start losing power. I had to run with the gas cap a little loose till I fixed the venting.
 
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prodiverr

New Member
Jul 8, 2015
6
0
0
Nanaimo
Hi. thanks for all the advice. It is fixed. Going for a test drive shortly to make sure. It was a vacuum lock. The gas cap needs to be replaced. the spring is not good and it created a vacuum lock somewhere. I got the cap sealed and had to squeeze the fuel line a few times to get the lock out. Now it seems to be running fine. Something is simple, but so hard to track down sometimes. You wouldn't think a small air leak like that would create such a big problem.
 
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prodiverr

New Member
Jul 8, 2015
6
0
0
Nanaimo
here is an quick update. I put the high end gas in 91, and that cleaned the gas tank of some loose particles. I seen these particles in the gas line to carb and stopped it right away. So glad I didnt clog the carb intake jet. Took out all the gas, cleaned out and rinsed many times with new gas. Put in new high quality 91 gas , and just got back from a 20 km ride at 50 kms an hour, now runs perfectly.
 
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prodiverr

New Member
Jul 8, 2015
6
0
0
Nanaimo
Yes, this is very good advice, I think next time I will use the mid grade. The high grade is making it run better, but hotter.

Thanks


Davezilla said:
Another thing to consider with these small engines is that they shouldn't need higher octane fuel unless something was done to give it higher compression. Most engines aren't that picky but some of these small engines can be pretty picky as far as fuel and octane goes. Octane is the fuel's resistance to burning so switching to a higher octane could give it problems where it just don't feel quite as strong as it should, or it could even hesitate or miss depending on how strong or weak the ignition is.
The basic rule of thumb with the fuel as far as octane goes is to use the lowest octane you can without pinging or detonation. I know high octane are advertised all over the place as premium or that this fuel will give the engine just a little more power, but that's just the oil companies way to sell more of it. Think of octane more as a necessary evil when you got a high compression engine where you need to tone it down in order to prevent detonation, kinda like retarding timing or reducing compression or adding octane.... so basically it should be avoided unless needed.
My engines tend to start easier, run smoother, and typically have just that little bit more pep to them when just running regular 87 octane pump gas. I do have to run 93 octane in one engine since it has a 12.7:1 compression ratio but the engines that are a lot closer to stock definitely run better with the lower octane regular gas.
If your engine is stock and if it's picky enough, it could just be the fuel that's giving you the problems.
Click to expand...
 
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prodiverr

New Member
Jul 8, 2015
6
0
0
Nanaimo
prodiverr said:
Yes, this is very good advice, I think next time I will use the mid grade. The high grade is making it run better, but hotter.

Thanks
Click to expand...
And its not a stock engine. the gas intake is triple the size of normal stock engines. Its a custom built, welded frame. It is not a power assist setup.

JJ
 
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