Hello fellow riders.

This is going to be a rather extended post, I want to give all the details of my problem so please bear with me. Also, please take in consideration the fact that I`m not a native English speaker.

I got my VTR1000 Superhawk `97 four months ago and a month ago problems started to appear - loss of power, overheating and oscillating idle speed. Took it to the shop and found out that my thermostat was faulty and one of the exhaust valves was leaking which caused my no.2 cylinder to flood with petrol overnight.
After a compression test I found out that the rear cylinder had only 4.3psi (normal 12-13psi, europe manual). I`ve replaced the thermostat and the exhaust valve and thoroughly inspected the engine – no other problems were found. Changed the oil, oil filter, spark plugs and cylinder gasket. Compression test shown 12.8psi on both cylinders.
After thoroughly cleaning and synchronizing the carbs, I took the Hawk on a test ride. The overheating was gone, but idle speed was still oscillating and the power was nowhere near where it should be.
I measured the compression again; front cylinder 12.8, rear cylinder 6.2. Took the carbs off and measured again – 12.8 on both cylinders. After going through the posts on the forum, I decided to play with the vacuum hose, and linked the vacuum from my rear carb to my front carb. Psi reading now shows 8.9 on my rear cylinder.
I took it on the road again, and for the next 3 days, I played with the mixture and idle settings. Finally I got it close to where it should be. Yesterday, after a few rainy days, I decided to take the Hawk on the road again but it wouldn`t start. The engine turns over but it would start. Finally with allot of help from the throttle, the engine started. After warming up, idle shown 900rpm. Adjusted it back to 1250 and took it on the road.
First off, the power was gone. 2nd gear, throttle fully opened, I couldn`t get the rpm above 5000. I decided to turn back and head to the shop. Half way there, rpm suddenly spikes to 9000 and everything`s back to normal. By the time I got to shop everything was running smoothly. The mechanic took it for a ride, no problems. Ok…again, I take it for a drive. No problems for 5 or 6 k. By now the engine is hot and the fans kick into action. No worries there, since is very hot outside. I stop at a traffic light, put it in neutral and wait. Few seconds later, I hear a powerful thud from my carbs. Felt like someone was punching my ***** trough the seat. Rpm spikes to 6000rpm, engine dies. Engine starts again easily, rpm shows between 1110 and 1400, no apparent loss of power. Second stop light, same symptoms – thud, engine dies. Downhill, using engine brake I`m starting to get allot of backfire.In neutral, the rpm drops to 900 and then the engine dies. I decided to readjust the idle speed, took it back to 1250. Engine runs normally on neutral. Another few hundred meters, another thud, but this time rpm jumps to 3600 and stays there even in neutral. This is where I sopped tinkering with the settings, and took the Hawk back home. Haven`t started it since yesterday.
I`m lost here. What am I missing? The engine has only 20000k on it; the bike is in excellent condition (carbs aside).

To sum-up the symptoms:
- Massive loss of power (problem corrected itself)
- Backfire while going downhill (using engine brake)
- Explosions in carburetors (followed by engine stop or idle speed spike to 3600)
- Idle speed drops after engine warms up causing the engine to stop
- Engine won`t start without throttle help (when cold, choke on)

Carburetors thoroughly cleaned (including pilot jets) and synchronized (4 times in 1 month), fuel tank removed and properly cleaned. Fuel and vacuum lines inspected.

 

Petcock... That's my bet... Either the diaphragm is ruptured, or you have the vacuum line on wrong...

Could be that you have other issues as well, but start by checking that the petcock flows with vacuum on the line, and stops when the vacuum is cut... And that it's on the side pointing nipple on the back, with the nipple on the bottom empty...

 

UPDATE:

Took the Hawk back on the road. Needed help from throttle to start, but after a few seconds, engine runs smoothly. Everything is as it should be, except that every now and then, feels like the engine is choking. The mixture is a bit lean. Due to heavy rain I was forced to take the Hawk home, before I got the chance to warm the engine. Thinking back, I realized that problems start as soon as the engine is warm enough for the fans to kick in. Forgot to mention, a few days ago, I removed the thermostat to resolve the overheating problems.