Working With the LT80 Carburetor
The carburetor situation on a Yamaha LT80 is straightforward once you know what you are looking for. These bikes use a Keihin PE17 or PE18 carb depending on the model year and market. The diagram you find online usually maps out the jetting passages, float chamber, and main circuit. What matters more than the drawing itself is understanding how those passages actually interact when the bike is running. When people search for a Lt80 Carburetor Diagram, they usually want one of two things: a visual reference for rebuilding, or a jetting guide to diagnose running issues. The diagrams themselves are pretty standard across Keihin PE-series carbs. You will see the needle jet, main jet, pilot jet, air correction jet, and the emulsion tube all laid out. The float level spec is around 17 millimeters measured from the gasket surface to the top of the float with the carb inverted. That number matters more than anything else on the page. I spent an afternoon last winter rebuilding a friends LT80 carb that had been sitting for three years. The vintage diagrams I found online showed the parts order correctly, but they did not mention that the pilot jet on these Keihins tends to get pinned shut by varnish in a way that is almost invisible to the naked eye. Standard compressed air blowing does not always clear it. I ended up using a strand of fine wire pulled through the tiny orifice to clear it completely. That fixed a lean idle condition that had been driving us both crazy for about forty five minutes.
The float needle seat is another area where factory diagrams can be misleading if you take them literally. The seat itself is a small brass insert pressed into the carb body. Over time, the needle tip wears a groove into it, and the carb starts leaking fuel into the crankcase. You will notice a strong gasoline smell in the oil. The workaround is replacing the needle, but also inspecting the seat with a magnifier. If it has even a slight groove, replace the seat insert as well, or the new needle will seat poorly within a few hours of riding.
What the Diagram Actually Shows
A proper LT80 carb diagram breaks down into several distinct circuits. The idle circuit runs from the pilot jet through the emulsion tube and mixes with air before entering the intake manifold below the throttle plate. The main circuit takes over around three quarters throttle and uses the main jet combined with the needle jet and sliding needle. The transition band between these two circuits is where most tuning problems live. The needle position on an LT80 carb is controlled by a clipslot system. Three positions are typical. Position one from the top gives the richest mixture, position three is leanest. The stock setup usually runs in the middle clip slot. If you are working at altitude, moving up one slot is the standard adjustment. If you are at sea level and the bike is hesitating off idle, dropping down one slot often helps. This is basic stuff, but it is also where beginners waste the most time chasing problems that are actually just needle position issues. The air correction jet is small and easy to miss on a parts diagram. It is located in the emulsion tube assembly and controls how much air mixes with the fuel at mid-range. Most people ignore it, but if your LT80 surges between twenty and forty percent throttle, checking whether this jet is clogged or was replaced with an incorrect size is worth doing before you start changing everything else.
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Common Misreads on These Diagrams
Many free diagrams circulating online have the jet sizes wrong. I have seen three different versions of the main jet specification floating around, ranging from a #75 to a #95. The correct stock main jet for most US spec LT80s is a #80 or #82 depending on year. The pilot jet is typically a #40 to #45. If you are buying a rebuild kit, check the actual jet sizes inside rather than trusting the diagram printed on the packaging. Another frequent issue is the float height specification being listed in inches on some diagrams and millimeters on others. A common mistake is reading 17mm as 17/32 inch, which is actually about 13.5 millimeters. That difference will make the float bowl run either too rich or too lean across the entire RPM range. Always verify the unit of measurement before adjusting. The gasket surfaces on these carbs are deceptively simple. The diagram will show a single base gasket, but in practice you often need to replace the cork or paper gasket between the carb body and the intake manifold as well. Old gaskets compress unevenly and cause vacuum leaks that no amount of jetting adjustment will fix. If you are doing a full rebuild, replace every gasket in the assembly, not just the ones the diagram highlights.
Where to Find a Reliable Diagram
The most accurate diagrams come from Yamaha service manuals or Keihin parts catalogs. The official Yamaha LT80 service manual has exploded views with part numbers for every single component. Aftermarket sources like Motocross.com or eBay sellers sometimes have scanned versions available. Free diagrams on generic carburetor sites are usually low resolution and occasionally contain errors in jet sizing or part ordering. I tend to keep a scanned copy of the Keihin PE17 service bulletin on hand rather than relying on crowd sourced images. If you are looking for a downloadable reference, the Keihin PE-series technical manual is freely available through motorcycle forums and archives. Search for the PE17-17 or PE18-18 technical sheet. Those documents contain the actual orifice diameters and passage dimensions rather than just pretty pictures. That is where the real tuning information lives.
Practical Notes From Real Work
The biggest practical issue with LT80 carbs is not the diagram but the condition of the existing hardware. These bikes are decades old now. Rubber diaphragms crack. Springs lose tension. The throttle shaft bushings develop play that creates a vacuum leak the diagram cannot show you. When I pull one of these carbs apart, I always measure the throttle shaft play with a dial indicator before cleaning anything. If there is more than point zero zero two inches of play, no jetting adjustment will give you consistent results because the vacuum signal is inconsistent. A second thing nobody puts in the diagram is the effect of intake manifold condition. The rubber intake manifold on an LT80 hardens and cracks over time. Cracks create unmetered air that leans out the mixture at mid throttle. I have seen people swap jets three times trying to fix a surge that was actually an intake boot problem. Inspect the manifold for hardness and surface cracks before you start touching the jets. The carburetor itself is not a complex device, and the LT80 version is no exception. The diagrams help you understand the layout, but the actual work happens in the details that those diagrams omit. Float height, shaft play, gasket condition, and intake sealing are all factors that determine whether your bike runs correctly. Focus on those first, then use the diagram to guide your rebuild. That sequence saves a lot of trial and error.
