Engine runs on only two cylinders

grahamcp

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I was having problems with the engine missing under acceleration, that eventually deteriorated to running on three cylinders. I took it to a Honda shop in town and the mechanic said it needed a valve job. I wasted $1700 on that. It ran the same, but added backfiring to it. I tightened the gaskets and the backfiring stopped. I checked all the electrical components and they were working fine. I took the carburetor off and had it rebuilt. After reinstalling it, the engine will start but runs only on cylinder 1 and 2, whereas before it was only number three that was not firing. There is spark to all the plugs. Where should I look next? The bike is a clean, 1982 Custom 750 with only 17K+ miles on it.
 

dmann707

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I was having problems with the engine missing under acceleration, that eventually deteriorated to running on three cylinders. I took it to a Honda shop in town and the mechanic said it needed a valve job. I wasted $1700 on that. It ran the same, but added backfiring to it. I tightened the gaskets and the backfiring stopped. I checked all the electrical components and they were working fine. I took the carburetor off and had it rebuilt. After reinstalling it, the engine will start but runs only on cylinder 1 and 2, whereas before it was only number three that was not firing. There is spark to all the plugs. Where should I look next? The bike is a clean, 1982 Custom 750 with only 17K+ miles on it.
 

dmann707

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you need 3 things to make run ....ignition /spark. fuel and air, compression...do a compression test see if all the cylinders are within a few psi of each other....with a new valve job they should be close....valve clearances should be around .005 inch or so... since 1 and 2 are firing the coils and ignitors are working check the wires and plug caps next ....copper core wires and 5 k resistor caps are standard here... ngk makes new ones...test for spark on all cylinders....next is fuel and air, make sure all jets are open and floats and needles are clean...make sure you have a stock airbox or getting it to run right will be difficult if running pods...finally change sparkplugs to new ngk d8ea is stock ...if all this checks out . it should run...
 

grahamcp

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you need 3 things to make run ....ignition /spark. fuel and air, compression...do a compression test see if all the cylinders are within a few psi of each other....with a new valve job they should be close....valve clearances should be around .005 inch or so... since 1 and 2 are firing the coils and ignitors are working check the wires and plug caps next ....copper core wires and 5 k resistor caps are standard here... ngk makes new ones...test for spark on all cylinders....next is fuel and air, make sure all jets are open and floats and needles are clean...make sure you have a stock airbox or getting it to run right will be difficult if running pods...finally change sparkplugs to new ngk d8ea is stock ...if all this checks out . it should run...
The spark plugs are new and spark when grounded and turning the engine over so I'm confident that the electrical system is working fine. I've done all that except checking the bowls for gas in the carbs after the rebuild. I'm not sure how to check that the jets are all open. The rebuild was done with a new kit so all the parts should be clean. I did not take the carbs back apart after the mechanic rebuilt it. What's curious is that it's both cylinders on the same side. It was only number three cylinder that wasn't firing before the carb rebuild. Is it possible that the floats hung up during the difficult install? Putting the carb back on was quite the feat. The rubber boots on the stock airbox are hardened and difficult to manipulate back on the carbs. I did not pull the engine. Do they still sell new ones? If the new ones are flexible, the install would go a lot easier.
 

dmann707

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yes i think u can still find new rubber airbox boots ...and new ones make the install much easier...
ive had floats get stuck when refilling from empty tap the bowls with a screwdriver handle or open the drains and check that gas runs out of all carbs
 
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