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1985 VN-700
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Blowback reminds me of my wifes 71 Dodge Dart slant 6. It was idling rough as heck one time. I was a youngster still learning. A former Dodge mechanic at work said bring it to work. He switched around two plug wires. Said crank it. It backfired through the carb. Ok. He swapped the two wires back to the right place and it idled smooth as silk. Have to do that every now and then he said. Carbon. But of course those were the days of leaded fuel.
 

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I’m just now seeing this. Reviving an old thread here! Going to give the back blow a try this weekend. Can it be done with a compressor? If so, what PSI do you all recommend? Also need to check the petcock. Not sure it is functioning as it should. Any advice on how to check it’s functionalitg?
 

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I test the petcock by using a pair of 11" needled nose pliers to clamp the vacuum hose(middle hose coming straight down from petcock) just below the nipple on which it's attached and wiggling and pulling straight down. I don't touch the hose clamps because doing so just warps them. Reinstall the hose in the same manner.
Then remove one of the fuel hoses which are the two hoses mounted to petcock nipples right and left.

Then attach a plastic syringe to the vacuum nipple on the petcock with a rubber hose or purchase a hand vacuum pump kit from an autoparts store. Pull the syringe or vacuum pump and if petcock diaphragm is good, fuel will begin to flow from open petcock fuel nipple. Turn off petcock to stop flow.

It's best to install a long rubber hose from petcock fuel nipple to a container to catch the fuel for safety and less mess.

If you use a vacuum pump, it's not necessary to pump it hard to get fuel flow. Pulling too much vacuum could damage the petcock diaphragm.

[*edit:

Better yet, just remove the petcock vacuum hose off of the left carburetor nipple with the needle nose pliers. Insert the syringe into that hose.]

I used 150 psi for backblow through the carburetor drain nipple. I might not should have but it didn't hurt mine any. Others may have an opinion of best pressure.
 

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May sound stupid here, but this is what this forum of for. The experienced to help out the ill-experienced!

Could I test the petcock using your method, but leave the drains open on the carbs? My thinking...I’d save time by leaving the two fuel lines from tank on carb but I’d pull the vacuum line (center hose) off at carb end, use syringe to pull vacuum on petcock. Gas would flow into carbs but drain out as they filled almost as a flush. Am I thinking correctly, or will I lose fuel out of the open vacuum port on the carbs as it filled up?
 

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May sound stupid here, but this is what this forum of for. The experienced to help out the ill-experienced!

Could I test the petcock using your method, but leave the drains open on the carbs? My thinking...I’d save time by leaving the two fuel lines from tank on carb but I’d pull the vacuum line (center hose) off at carb end, use syringe to pull vacuum on petcock. Gas would flow into carbs but drain out as they filled almost as a flush. Am I thinking correctly, or will I lose fuel out of the open vacuum port on the carbs as it filled up?
I have always tested my petcock with the tank off the bike and that's what I was seeing in my head as I was describing it to you.

I see no reason why you couldn't test it thru the carb drain.

I strongly recommend that you use a drain hose into a container. A puddle on the floor produces a lot of fumes which could be accidently ignited. I know a guy who was badly burned and missing an arm from fumes igniting in a closed shop.

You could do one carb at a time.
 

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May sound stupid here, but this is what this forum of for. The experienced to help out the ill-experienced!

Could I test the petcock using your method, but leave the drains open on the carbs? My thinking...I’d save time by leaving the two fuel lines from tank on carb but I’d pull the vacuum line (center hose) off at carb end, use syringe to pull vacuum on petcock. Gas would flow into carbs but drain out as they filled almost as a flush. Am I thinking correctly, or will I lose fuel out of the open vacuum port on the carbs as it filled up?
I have always tested my petcock with the tank off the bike and that's what I was seeing in my head as I was describing it to you.

I see no reason why you couldn't test it thru the carb drain.

I strongly recommend that you use a drain hose into a container. A puddle on the floor produces a lot of fumes which could be accidently ignited. I know a guy who was badly burned and missing an arm from fumes igniting in a closed shop.

You could do one carb at a time.
Well. I was able to drain the carbs. Looked like Mountain Dew coming outta there so I know I need a good cleaning. Also I was able to test the petcock. Seems to be functioning with vacuum. The right right side carb seemed to flow faster that the left side, but they both were getting fuel. Next step is to fill carb with seafoam. I investigated this today and it seems very difficult to get to these hoses with the tank on so i am going to get the tank off so that I can do the Seamfoam soaking and then the blowback a coup times. No hurry as the weather here in PA is changing. How long is too long to let seafoam sit in the carbs? Can I put it in and wait a week or should I drain it the next day or two?

I sure hope this blowback trick solves my problems. The color of the fuel that came out has me concerned!
 

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Honestly, if there's that much crud, you're better off going inside and cleaning the carbs. There's going to be stuff that seafoam won't dissolve (will look like grains of sand), and the blowback is just moving it around.

I'm not saying blowback is bad, but it's more of a trick to get you off the trail and back home, instead of a permanent fix. Doing the blowback, just leaves the crud to plug up the carb again later.

How long to leave it in, depends how quick seafoam evaporates. I don't know the answer. Maybe a day or two.

Looks like tomorrow is our warmest day this week, 62°.
 

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@Stvz and Spockster:

Yeah, after my bike sitting for six years or so, my fuel drainage was quite clean and I tried the backblow several times with no success. Wound up doing a full ear shave and thorough carb clean, then experimenting with various jet sizes.
The carb clean was the only thing that got my bike running above 50mph.

I've done the carb removal and jet swap so many times now that I've got it down to an hour and a half back to full running.

I just went from 40/140 to 40/142 for a couple of days.

The 142 finally changed my plug color to a nice tan/brown which the 140 never did, but although the 142 made it run like a scalded dog, I couldn't get rid of all the popping and fast fuel consumption. So, I swapped it back to the 140 which runs good, cool, fast enough, 50mpg, and scarce popping. I figure it's close enough away from the extreme lean condition which I had that it's safe enough and will likely leave it at that. If I was inclined to race off the line, I'd definitely put in the 142s. Man, what power!

I wouldn't hesitate to earshave if I had one that required carb removal. I wholeheartedly recommend it.
If one goes to all the trouble to remove all the EPA stuff and then to reinstall it just to get to the carb removal, what if you have to turn around and get right back into the carbs again for some reason, which is very likely. I just wouldn't do it. I'd go with the full earshave and render the bike so that I could access the carbs easily at any time and have it running again within a couple of hours rather than most of a day or more.
 
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